Hi James, my girlfriend and her sister have Fitbits and they enjoy setting challenges to each other and seeing who is winning each time etc. They seem like great devices and have great battery life. The one my girlfriend has is the Charge HR, and its battery lasts like a week on a single charge. But she only uses it for step counting mainly. The Fitbit app on Windows and Windows Phone is great and really full-featured - I imagine it would be the same if not even better on other platforms. the only problem she has had with her Fitbit is getting it to sync reliably on different devices. She had to have her Charge HR replaced, so it could be just a faulty device though.
I, on the other hand, have a MS Band 2 and I've had it since Christmas. I've been using it for checking emails, text messages, Cortana reminders and appointment reminders etc as well as keeping track of the quality of my sleep and daily steps etc. I even go for runs with it and it's been really great to see the device keep track of pace and GPS etc. You can even take steps like telling it to track the GPS less often if you are going for a longer session, in order to maintain battery life. I also use it to control music on my phone daily, and it's been amazingly handy for that. I get regular device feature and stability updates, and I'm really enjoying using the smart alarm feature, which wakes you up within a 30 minute window of the time you set when you're the most alert. I've really noticed the diference there.
I don't use the MS Band 2 for any social competition or anything as I don't know anyone else that has one near me, and I can't compete against my girlfriend (because she has a fitbit) - but I'd love to see some inter-device social networking there although I'm not sure that's ever going to happen. I get 2 days use out of my Band 2, and really it's just a case of making sure to charge it for 30-40 mins in the morning to make sure you're going to get through that day -- then you never have to specifically set aside another time to charge it. It's been quite comfortable for me to wear daily, and it's features really work well (love using Cortana through it!)
If you are going to track any exercise or a run, though, make sure you charge it the next morning in order to get you all the way through the next day.
Let me know if you have any more specific queries and I'll get back to you.