I don't have one, but I've researched it, and I've been interested in getting one.
It's similar to a bodybugg, but appealed more to me since you don't have to wear it on your arm. It's a more discreet, because you either wear it in your bra or on your waistband. I also like that the website membership is free for the fitbit, whereas the bodybugg membership has a monthly fee. I've been successful just using fitday to meet my weight loss goals, and managing my calorie deficits, but I'm definitely considering getting a fitbit to help with my exercise goals. I've been working out fairly regularly, but it comes harder than a clean diet for me, and every bit of motivation helps! Also, working graveyard shifts only part time wreaks havoc on my sleeping habits, and I'm very curious how much REM sleep I average on a given day. My hope is that I could make adjustments to my lifestyle to help my sleep patterns be more consistent.
It's nice to hear feedback from someone who uses fitday as their food database. I was wondering how I would do that, because I want to continue using fitday... I have four years worth of data and custom foods entered, and I don't want to start over with a different program!