I recently picked up a Fitbit Zip at a Radioshack store closing for a heck of a good deal with the idea of giving it to my 2 year old son to see just how many steps his short legs make in a day. Obviously he didn’t need a One, or a wrist band style Fitbit, so the Zip seemed perfect. It still clips to clothing, and the battery reportedly lasts about 6 months (a CR2025, I believe), and best of all, it syncs over BLE (Bluetooth Low Energy). In the first few days I had the unit, it counted well over 8,000-10,000 steps a day (often beating both my wife and myself). The only issue I found was that it didn’t Sync quite as reliably as the Fitbit One. It seems that in order to get the 6 month battery life, they have the unit go into standby after a bit of inactivity. So during the night, when we’d remove the Fitbit from his clothes, it wouldn’t sync. I had to actually place the unit much closer to the receiver to get it to sync during the night (I’d just leave it on my desk which is where the receiver is). Other than that issue (which will come back) the unit is great. It’s small, does a good job, and we loved it.
Then, sadly, the unit seems to have fallen off him when we were out shopping one day. And because the unit goes to sleep with inactivity, you can’t use something like Lightblue Bean (or any other BLE scanner) to find it. It just happily sleeps away, with no ability to say “oh, it’s in this store/parking lot/etc”. Sadness. I’ve since contacted Fitbit to see about a replacement, but at this point, I’m unsure. Hopefully a new one will show up, and we’ll get back to counting. =)
As a product though, it’s great. I’d suggest one in a minute for just about anyone. And if Fitbit could update their firmware to have the unit wake up once a minute or so, just to “ping” a BLE signal, so that you could see if it’s anywhere near you, that would be a great addition for finding the unit. Sadly, I think someone picked it up and didn’t turn it in to lost and found. =(
[xrr rating=4/5]
UPDATE (5/11/2015): I just received a replacement Fitbit from Fitbit in the mail. Awesome job guys, thanks so much!
Two years ago, I purchased an Oregon Scientific Pedometer off REI Outlet as a “trial” for whether I would actually use a Fitbit, and therefore justify the price of a Fitbit (which I had seen and heard about from a colleague for years). Initially I only planned on using the cheap one for a couple months, then upgrading, but a couple months turned into two years. So, I purchased a FitBit One for my “christmas” present, and just started using it. The review will be rather short, since it’s a pedometer, but all and all, I’m extremely happy. The unit is half the size of the Oregon Scientific, so it still fits in my coin pocket. It’s rechargeable, and while the specs say it should last about 10 days on a charge, mine seems to last about 2-3 weeks between charges. The screen is OLED, and shows steps, distance, floors (the unit has an altimeter), time, and some flower I haven’t yet figured out. 
I recently got a FitBit One (review forthcoming), and one of the big reasons I got it was their open API. Basically, the FitBit will sync over Bluetooth 4.0 (Lower Power) when it sees it’s Dongle, or if you have an iPhone/Android phone, it can sync to that in the background. That data syncs up to the FitBit website, at which point you can pull the data back down using their API… at least, that was my hope.