My old Fitbit, which I think they call the Fitbit Classic, but looks like the Fitbit Ultra, started being problematic at least 6 months ago. Even though it had a full charge, it would randomly and suddenly not track steps and I would have to plug it back into the charger to have it work again. Once or twice I even had to press a button on the charger to get the Fitbit to "reset" itself. Then, starting in September, the Fitbit would constantly not track steps, even though plugging it into the charger would show that the Fitbit was charged. It was becoming annoying. Plus, the holster for the Fitbit had chipped off a bit a few months before hand and had a sharp edge. It was time for a new Fitbit.
A friend of mine has the Fitbit Flex, but as I already wear a watch, I really didn't want to wear more than one thing on my wrist. I could have gone with the Fitbit Zip, but was more inclined to go with the Fitbit One, as it would also track the number of stairs that I take. The sleep tracking capability really isn't important to me. I used it once on the old Fitbit, but trying to wear something on my wrist as I sleep is too annoying.
The Fitbit One has some improvements over the Fitbit Classic that I like - the charger is a shorter dongle that is much easier to pack than the old one. The holster is more of a soft rubber sort of material - similar to the OtterBox I used on my old Samsung Droid Charge phone. It can be a bit challenging to get the Fitbit completely in there at times and I wonder if the holster will become very loose to where it does not hold the Fitbit as well as it does now. Removing and reinstalling the OtterBox on my phone caused it to become looser. So far, after a month of use, I have not seen that problem.
The Fitbit One is supposed to sync via the Fitbit app for various Android phones. I haven't gotten it to work yet on my Moto X or Nexus 7, but another page (which is not easy to find) listed a problem with the Bluetooth drivers on Motorola devices and that a bug was submitted for Nexus devices. I hope this gets remedied soon, as it will be nice to sync my steps if I am away from my computer for days at a time.