I have one and use it daily. The amount of data you can track with the FitBit is great, as I love to see exactly what I am doing physically throughout the day.
The wireless uploading is great, and my favorite feature is the sleep tracking. Nothing like finding out your sleep cycle, and being able to view it in a shiny graph.
The website interface is decent, though there are some slight bugs in random corners. I have not yet tried the iPhone app that is apparently a nice compliment to the device itself. There is a calorie tracker built into the website, but also offers integration with many other sites (I use myfitnesspal).
Its very simple to set up initially, and I’ve gotten into the habit of wearing it daily, no matter what I’m doing (minus swimming). Its small and easily concealed, so its not some crazy big device that is immediately noticeable.
Battery life is decent, I generally get about 5 days out of it before I need to charge it again.
The pricetag of $99 didn’t seem to bad, and I now consider it well worth the cost for the amount of use I get out of it.
I would highly recommend one for those tech-centric users who love to integrate fitness and technology.