Polar is following Garmin's footsteps, in that it started with more fitness-focused devices and is now venturing into the health and wellbeing domain, with the hardcore fitness bases still covered. It's about how fitness and sleep fit into the rest of your day, rather than separating your life into isolated chunks.
To do this, Polar brings all-day heart rate sampling to the A370, which the A360 didn't have. If you're not exercising, the tracker will take a sample every five minutes of the day - but will move up to once per second when you're in a workout. This is something you can choose to switch on or off, but having it on means you'll get some juicy graphs telling you when you had your highest and lowest heart rate through the day, and pinpoint your lowest heart rate reading during sleep.
I'll get into the meat of the heart rate tracking shortly, but for day-to-day measuring I tested it against different chest straps (Polar H10 and Wahoo Tickr X) and other wearables like the Charge 2 and Apple Watch, and found the numbers to be spot-on. So the data you're getting on these is reliable in that respect at least.
The A370 does quite a few things, but the interface is surprisingly bare; I quite like it. You can see an overview of your day on the display, which shows you your step progress for the day, details on any workouts you've done, and, again, your peak and lowest HR recordings.
As is custom with most trackers now, there's an option to check your current heart rate. Then there's an option to launch a workout, and Settings. And that's it. Polar's keeping things simple here, and I've found the experience better for it.
Boot up the workouts and you've got a menagerie of activities to pick from, from running to aerobics to strength training to hiking. Note that in all these cases it's just letting you customize heart rate zones and GPS differently; it won't start measuring things like reps or cycling cadence unless you pair a separate device. You can use custom-built Polar workouts though, which you'll need to do on Polar Flow and then sync to the tracker.
Here's something the A370 doesn't have: built-in GPS. But, like Fitbit, it has a connected GPS which uses your phone's signal. In practice, I found this less stable than I expected. I thought that if anything it would make for a stronger connection, even if it meant I had to take my phone with me. But I found that I kept getting a buzz on the wrist and a "GPS signal not found" message on the display. Often this was when I was running in more built-up areas, but we're talking streets of houses, and even then I've had plenty of other wearables keep their GPS lock OK in the same areas.