I was most interested in the sleep measurement, and was very impressed by how well it works. You get a diagram of REM sleep, deep sleep, light sleep and alertness. It is, of course, difficult to know how precise it is, but it seems to be reasonable with your own experiences of alertness and dreams. Sometimes, however, the sleep is missed by 30 or 60 minutes, so that reported sleep time is thus too short.
The heart rate measurement works like this. Very good with heart rate monitor with chest strap at low to medium pulse, but at high pulse Fitbiten consistently shows 10 too much.
Synchronization and charging work well for the most part. Battery life quite ok.
The worst thing is the bracelet. It slowly slows, now after 5 months with 1-2 holes. And now the bracket on the watch begins to give up. Must already buy a new bracelet.