I got a Fitbit Flex for Christmas and so far I really like it. I would prefer the Force, which has the clock. The Force also uses and altimeter to gauge stairs climbed.
The Flex (all of the Fitbits, I believe) are advanced pedometers that calculate your steps/distance based on your age, height, weight and movements plugged into some sort of algorithm.
You need a computer to set up your device, after that you can either use your computer to sync or sync using a a smartphone, tablet or other device. The key is the device must have and be able to use bluetooth 4.0. (my old droid phone and 2012 nexus tablet for example, do not have 4.0 and will not sync to my Fitbit. newer androids and the 2013 nexus will work. There is a list of compatible devices on the Fitbit website.) All your synced info, plus community forums are accessible for free on the website.
I haven't had any problems with mine, although its only been 2 weeks. I do find it motivates me to try and move more; I like seeing the numbers on my charts go up and the lights blinking on my Fitbit
Oh, and it links with MyFitnessPal, which I like. You can track your food on the Fitbit site, but I have been using MFP for a year, so I like that I can stick with that. It appears to sync data input to MFP instantaneously to the Fitbit Dashboard.