Picked up a Fitbit One last week (and by "last week" I mean May 2014) - partly because a bunch of my friends got them (and not just the ones who actually work there now :-) Instead, enough other people got them and started talking about answers to questions I hadn't realized I wanted to ask.)
In particular, even though it's "locked down" hardware, it's very well designed - slips into a pocket, doubles as a sleep tracker, works with my android phone, haven't needed to recharge it yet (seriously, I get 10ish days on a charge, though I only sync it intentionally once or twice a day, not automatically) - and generates convincing data - and is cheaper and easier to use than many more open but less polished alternatives. (That might be the subject of a later review; this post is just about the Fitbit One itself.)
Note that I got the Fitbit not for the competitive-tracking aspects, or the daily analytics, but because it turns out that it's easier than I'd realized to get in to their "partner" program and actually get higher precision data out of it. (However, half a year later and I still haven't actually signed up...)
I do have to give them credit for the design of the competitive parts, though; how many "social" apps do you use actually have, right at the top of the friends page, a "TAUNT" button? Also, the fine-grained control of data-sharing is a pleasant contrast to everybody else on the internet so I can share steps (and specifically "allow ranking" in addition to that) without exposing the data I don't really care about or that I'm still working through issues with.
It is a little tedious to put it into a wrist-sleeve and explicitly enable the sleep tracking; I've got a mostly-reliable habit of it now, but improvement in that one area (including automatically noticing naps, which I wouldn't otherwise track explicitly) is probably going to cause me to get a Fitbit HR when they actually ship.