Long review, but short story is that I like it.
I've been using a Garmin vivosmart HR for a week, comparing it to a Jawbone Up Move activity tracker (without heart rate monitor) and a Garmin Forerunner FR60 exercise watch (with chest-strap HRM). The Jawbone Up Move and the FR60 are entry-level models, but in many cases the data should be similar, so I wanted to see how they compare.
For smartphone, I use a Samsung Galaxy Note5 which I paired with both the vivosmart and the Jawbone (yes, you can do both at the same time).
Display: Nice, large screen compared to other tracker models in this price range. Sharp, reasonably large fonts. Backlight is adequate to read the screen in the dark. Display can be in portrait or landscape, which is very nice. I wish the screen were in color.
Comfort: No problem. I wear the vivosmart all day and night, except when I charge it. I wear it snug because I think that's better for heart rate accuracy, but it's light enough that I don't feel it's there. Unfortunately the band is not swappable.
Step tracking for walking or running: That is the easy part. All models agree within 5%. Even the Samsung Health and Google Fit apps on my phone give similar results. I didn't see a pattern where one model is consistently higher than the others.
Step tracking for indoor cycling: I do a lot of cycling, and this is where the Jawbone Up Move has a unique advantage. I put it in my pocket and it counts the spins. The vivosmart simply cannot do that because it goes on the wrist, although it estimates calories burned based on heart rate (see below).
Sleep tracking: The vivosmart has automatic sleep detection, but it errs on the high side, meaning it thinks you are sleeping even when it detects that you are still moving. When I took an early evening nap, it did catch me napping, so that part is good, but it tends to think I go to sleep before I actually do. I can edit the sleep start time to keep my sleep record accurate. It tracks both deep and light sleep, as well as movement during sleep. One bad thing is that it records only one sleep event per day, so if you sleep twice in one day, the second sleep will overwrite the first, without adding to it.
Calorie estimate: The vivosmart calculates calories burned based on heart rate (same as the Forerunner FR60), while I think the Jawbone Up Move does it based on steps and distance. This run is typical of what I get: vivosmart 185 calories, FR60 190, Jawbone 197, so agreement is fairly good.
Heart rate monitor: I seem to have gotten better results than many other reviewers. Over several days and exercises, I compared the vivosmart HR to the FR60 with chest strap HRM, which is usually the gold standard. The vivosmart was usually within 1-2 beats per minute from the FR60, sometimes lower, sometimes higher but very close. The charts are also similar, with the same peaks and valleys and "smoothness". I don't know if my success is due to the type of activity I do (running and indoor cycling) or due to the way I wear the vivosmart (I keep it snug so there is no light leak between the heart rate sensor and the skin). HRM was the main reason I went with this model, and it was a pleasant surprise.
Move reminder: The tracker vibrates if you don't move in one hour, except when you sleep. I like the fact that it is persistent. To clear the alert, you have to get up and walk around a bit. I don't know how many steps, but it's more than just standing up and sitting back down.
Stairs tracking: The vivosmart tracks how many times you use the stairs (going up and going down are counted separately). It under-counts by about 20%.
Battery: I went 4.5 days before recharging, and it still had about 20% battery life left. This was with always-on message notification and always-on HRM, so that agrees with Garmin's spec of 5 days. The battery meter is a bar, not a number. Recharging takes 1.5 hours from 20% to full.
Message notification: Very reliable, for calls, text and email. You can customize which notification you want on Android (not sure about iOS)..
Music control: Basic but it works well (play, pause, next song, last song).
Data sync: I have had no problem whatsoever in sync-ing to my phone (Samsung Note5). Sync is automatic and immediate every time I open the app on my phone. I also sync to a PC using Garmin Connect. What is strange is that while the phone and PC always agree, they have "rounding errors" from the vivosmart. For example the vivosmart may say that I have 260 intensity minutes, but the phone and PC will show 263 at the same time. This is not data lag because the difference will stay like that over many days.
Phone app: Data is detailed, perhaps too detailed. There are a lot of screens and menu options, so it takes a while to know where everything is. By comparison, the Jawbone app is simpler - the layout is cleaner but it doesn't seem to be as "technical" as the Garmin app. They are different, but I can't say that one is better than the other.
Overall, I am happy with the vivosmart HR, and fortunately I haven't experienced the HRM and data sync problems many other people have seen. For my wish list, I would put a color screen, a numerical battery meter, and interchangeable wrist bands (and GPS, without sacrificing battery life).