I have a fitbit Charge. I can tell you this: I've been to gym more in the last month I've had it than I did in the 6 months before I had it. It was expensive, but it's an excellent motivator for me. A lot of my friends and coworkers have them, so I have a little walking club at work where we go for walks everytime the smokers go for smoke breaks. I haven't lost an ounce of weight, but I eat like a garbage can, so that's not helping much, but I do feel a lot happier in general and my skin is clearer than usual.
The social aspects of the fitbit are fantastic as well. You can friend other fitbit owners through the cellphone app and set up challenges. I've currently got a "Workweek Hustle" challenge going on with about 10 other users (some I know, some are friends of friends who've been invited along). If you're competative, it's a great tool to prod you into walking more, just so you can beat someone else's step count.
It also doubles as a watch, which I don't own, so it's nice to have the ability to tell the the time without having to dig out my phone.
I don't have the Charge HR (which has a heart rate monitor on it) but a friend of mine does and its apparently more accurate at calculating calorie burn. The fitbit Surge has a heart rate monitor and a GPS so you can track your runs.
The fitbits are however walking/jogging/running focussed. It reads my steps fine on an elliptical and a skate machine, but I haven't tried it on a rowing machine yet, so I'm not sure what it'll log in that case. If you're a cycler, weight lifter or a swimmer, then this sort of fitness tracker might not be what you want since it is so step count focussed. But for a lazy slug like myself, it's an amazing motivator to get up off the couch and get moving.