I have a Fitbit One. It's great for seeing how many steps I've walked and whatnot. I chose it as it's more subtle than the wrist ones, I didn't like the idea of having something on my wrist all of the time, also I read reviews about the wrist ones being less accurate as we move our arms so much (someone did a comparison wearing both at the same time, and the wristband one counted steps whilst she had been sitting still but moving her hands in conversation).
I think they have recently brought out new ones, or will be soon, but I don't think any of the ones I read about would be particularly useful for running or gym classes (I don't know if they can count running steps as well as walking steps). There is a distance calculator too which could be useful. But you might be better off looking at dedicated running watches or trackers if that's what you'd want to use it for.
Each to their own but I think doing some Parkruns and seeing your times improve, or challenging yourself to a new/different race, or fitting into a smaller pair of jeans, are all much more motivating than a step-counting thing you strap to your wrist. And so many reports of them over-estimating calorie use.
I recently bought a fitbit zip; it was quite convenient as it's so small and you can clip it discreetly on your waistband, ontp your pocket, or bra.
It is quite motivating to be able to see your activity throughout the day, and it encourages you with each milestone you get for steps taken per day.
However, I am sending mine back; I found it really overestimated calories burned when walking, although it may have just been a fault with mine as others on the fitbit community have reported much lower walking calorie burns than mine. I also use MFP everyday, and found it didn't work well with it - MPF always gave me really inflated calorie allowances based on fitbit's adjustments.
A lot of people love fitbit though, so could be worth a try. Alternatively, have you tried a heart rate monitor for tracking your achievements whilst exercising? (Although I have tried a couple of these as well, and found them to work very intermittently, so I think I am giving up on fitness gadgets!)