I hava a fitbit charge2 (I used to have a GPS watch with a separate chest strap - but the heart rate was *waay* out (I wasn't getting my pulse rate up to 240 bpm after half a minute's gentle cycling without even needing to breathe faster, for example) and then the GPS refused to acquire a fix.
The charge2 relies on my phone's GPS via bluetooth, and the heartrate is pretty good. It's also convenient. You might notice that I haven't said anything about the GPS...
When I got it, last August, it was good, but in February there was an update to the app and since then (and with several app updates) it has been reading low on distance.
Most of my logging is in cycling to work, and after the update, I noticed that my 20km commute into work was being recorded as 15km. When I looked at the route on the map it looks correct on the ground.
Fitbit have improved the matter, but it is still not right - I recorded yesterday's walk with both strava on my phone, and fitbit via my phone, so both using the same GPS. Apparently the strava app thought I'd walked 21.7km, but the fitbit thought it was 20.7km. My commute distance seems to vary by about 2km in 20 (normally under). I think it might be when it monentarily loses the GPS, because I recently cycled on the Monsal Trail with a local scout group and the Strava app gave 10.5km, whilst the Fitbit gave 9km (there are tunnels, so GPS will have been lost on that route.
In short, fitbit is great apart from the GPS.
which is quite annoying