I am of two minds on the Apple Watch. On one hand, I think that it's the best overall smartwatch you can currently buy. And to that end, the Series 4 is the best Apple Watch to date.

After wearing one for months, it's worked really well. And yes, it's still my favorite smartwatch to use, because its smooth-running interface and tight iPhone-to-watch integration is better than what other smartwatches offer. But while this new model has a lot of refinement over previous versions, and a pretty great leap forward from older models such as the Apple Watch Series 1, it's not as tremendous a leap forward from last year's Apple Watch Series 3 as you might think. 

Read more: All of the WatchOS 6 updates | Complete guide to Apple's Health app | 18 devices that sync with Health App 

And the Apple Watch is no closer to being a clear must-have device than it was before, unless you value the possible benefits of new health features. For those looking for a fitness tracker, however, the S4 hasn't changed much over the S3 at all.

The 2018 model adds a nice slate of upgrades:

• Thinner design with bigger, almost no-bezel watch faces: Larger 44mm and 40mm displays fit into the same general case sizes that previously held 42mm and 38mm ones, respectively.
  
• Lightning-fast speed: The Series 4 is fast. Response time for nearly everything that doesn't require an online ping is effectively instantaneous. The improvement versus Series 2 and earlier models is particularly dramatic.
  
• Fall detection: The Watch will alert loved ones and EMS when you take a serious tumble (but Apple doesn't guarantee it, and it's not on by default unless you're 65 or older).
  
• Electrocardiogram (EKG or ECG): This feature amps up the Watch's already impressive heart-rate monitoring features with a full-on, FDA-cleared, on-demand ECG app that will check for heart arrhythmia, via 30-second heart rate samples.
  
• GPS battery life on runs is improved a bit (6 hours instead of 4, which isn't much, but it's something).
  

On the other hand, there are still a number of downsides.

• The Series 4 is more expensive, starting at $399 (£399, AU$599). Larger sizes, optional cellular connectivity and upgrading to steel case or fancier straps runs the cost even higher.
  
• Battery life remains stuck at about a day and a half to two days on a charge, less if you're using GPS, cellular or exercise frequently.
  
• No sleep tracking: Because the watch needs to be charged every day, that effectively dampens sleep tracking options. There are third-party apps, but nothing Apple offers as part of the core health experience.
  
• No always-on watch face: To save battery life, the watch face is dark most of the time.
  
• Despite a handful of new data-rich watch faces, the apps that use these faces are limited, and deeper customization -- including having a watch face store like rivals do -- remains MIA on Apple Watch.
  

To be clear, none of these missing pieces are futurist fantasies. Rival products from Fitbit, Samsung and others are starting to last for days -- with promises of weeks between charges (in more pared-down watch modes) made by Qualcomm's new chip that's powering upcoming Google Wear OS watches. And those same products mostly offer sleep tracking, dozens or hundreds of watch face options, and always-on timekeeping, too.

Update, Dec. 21: Added testing of the now available ECG app. The score has not changed. Otherwise the rest of this review, first published on Oct. 19, is unchanged.

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Get this model, stick with what you have, or look elsewhere?

New features like automatic workout tracking, walkie-talkie communications between watches, raise to talk (no more "hey Siri") and better support for music and podcasts are all welcome additions to the Apple Watch. But those are part of the 2018 WatchOS 5 upgrade, so they're also available on older Series 1, Series 2 and Series 3 models.

To that end, if you're not a workout junkie, or someone who's particularly interested in the safety aspects of fall detection or EKG-level heart monitoring, the Apple Watch Series 3 -- available starting at $279 (£279, AU$399) -- is still a worthy option. That model retains the standalone cellular option, too, if you want to occasionally go phone-free.

The Apple Watch Series 4 starts at $399 (£399, AU$599) for the standard model and $499 (£499, AU$749) for the LTE model. Models climb up from there depending on size, material (stainless steel versus aluminum), band choices and additional styles (Nike+, Hermes).  

Existing Series 3 owners shouldn't feel a rush to upgrade. And Series 1 and Series 2 owners should start with that free update to WatchOS 5, of course. But if you're looking for considerably faster speed, a bigger and more information-rich watch face, and better health and workout features, the Apple Watch Series 4 is a worthy upgrade.

However, iPhone owners looking for a smartwatch with longer battery life or sleep tracking should check out alternatives like the Fitbit Versa.

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Apple and ECG: Do you need it?

The most interesting new feature on the Series 4 watch, by far, is an FDA-cleared ECG app that can do more medically accurate on-the-spot heart rate checks. The Apple Watch is now an over-the-counter health device. But what that means for you may not be as sweeping as you'd think, even if it could help more people be aware of medical conditions they didn't know they have.

ECG is short for electrocardiogram, which US doctors usually abbreviate to EKG. The one-lead ECG in the Apple Watch works much like a third-party band I tested last year made by Kardia. It takes a stationary 30-second heart-rate recording by completing an electrical circuit between the back of the watch and your finger on the digital crown. The heart reading can see the sort of peaks and valleys you'd see on a heart rate readout at a hospital, albeit less accurate than you'd get there (see below). But it can check for heart-rate rhythm abnormalities, which you'll be notified of. 

But the ECG app has its limits. It's a key step up, but it's unlikely to be anyone's comprehensive heart monitor -- it isn't even claiming that it can be.

CNET's Vanessa Hand Orellana consulted with a doctor and compared Apple Watch's ECG app to a hospital EKG. I've been using it for several weeks, and it's been an extra feature I haven't needed to dip into much. But I've taken a number of ECGs just because, and odds are you'll do the same when you buy one.

• ECG isn't always on. It is, by design, a 30-second spot check that does a deeper ping on your heart rate than what the normal optical green LED heart rate sensor can read. It's made to check for one thing: abnormal heart rhythms. If the watch does notice an abnormal rhythm, it immediately suggests calling 911. If results are inconclusive, it suggests continuing to check and then calling your doctor.
  
• It needs to be activated via Apple's Health app. From there, the app guides first-time users to how to it works, and what it can and can't do. ECG readings are stored in the Health app, and can be sent to a doctor or shown during a doctor visit.
  
• The ECG onboard is a "one lead" ECG, which is only cleared to check for atrial fibrillation and arrhythmia only. It can't detect heart attacks, and it's not as comprehensive as a doctor's 12-lead EKG, which can detect a lot more. It's an early way to spot a possible problem.
  
• It's meant to be used only at certain times. Apple says to use it when you're not feeling well, or when the always-on heart rate detects a possible atrial fibrillation during its periodic checks.
  

That check for possible a-fib using the regular heart rate reader also works on Apple Watches going back to Apple Watch Series 1 when updated to WatchOS 5, but the deeper ECG analysis only comes with the Series 4.

ECG readers are also available separately, like Kardia's little mobile-connected ECG device you could buy separately, or Kardia's separately-sold watch band for older watch models that I tried last year. So you don't need an Apple Watch S4 for ECG. But it's a nice extra to have integrated into the Series 4 watch if you were already considering buying something like it, and it could be a key reason to upgrade for anyone concerned about heart health.

How key? That's hard for anyone like me to say. My fellow editor Vanessa spotted a possible heart arrhythmia when using hers. I haven't spotted anything yet as I've tried it. I have high blood pressure, and ECG does nothing to help me on that front. It will undoubtedly help people spot possible heart conditions, and it already has after just a few weeks of activation.

Other health features: Fall detection, low heart rate

The Apple Watch S4 can also detect falls, via its improved accelerometer, gyroscope and optical heart rate sensors.

Fall detection works via a combination of G-force impact and detection of arm and hand placement when falling, using the watch's updated accelerometer and gyroscope. Apple's testing and algorithms look for indicators that apparently only happen in "real" falls. After falling and detecting a fall, as long as the fall detection has been turned on in Apple Watch settings, the watch will call 911 via your phone or via its own cellular connection, and will then contact a designated friend or loved one to notify, along with your location.

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The new watch can also detect abnormally low sedentary heart rate, in much the same way that current Apple Watches can detect high sedentary heart rate. Either can possibly be a flag for medical problems, but not a guarantee that anything's wrong.

While it's great that Apple is pursuing new ways to explore health, these features come with caveats. Apple won't guarantee successful fall detection, meaning that the app specifically warns that the watch's fall-sensing may not work in every instance. I took a light, simple spill (as if I had collapsed, versus taken a sharp fall), and it didn't detect. Unfortunately, the slight uncertainty factor means it can't be a guaranteed monitor for a loved one that needs extra care.

And the heart rate monitoring may not necessarily spot all problems. Apple's heart rate doesn't scan constantly, like Fitbit's trackers do, but it does check in every few minutes.

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My favorite thing: Speed

The first Apple Watch in 2015 was slow. Loading apps was its limiting factor. The lightning-quick, smooth feel of the Apple Watch Series 4 almost feels like a given, since it's how Apple products tend to feel out of the box. It's as fast, now, as an iPhone or iPad. It feels automatic. Last year's Series 3 was also fast, and it's still fine. The Series 4 does it one better, so that loading times no longer seem to exist much at all.

Speed is probably more key on a watch than on a phone, if you're counting on quick glances and instant info, I can't think of a better wearable.