I bought it, and I love it - but I'm a sucker for technology. The main thing I like is that I can download all my info, and add all kinds of insane statistics to it. I take the reports I create from the download to my surgeon every month to show him exactly what I'm doing. He doesn't really pay much attention to it, but he gets a kick out of it, and he at least knows I'm trying and I'm committed. The other thing I really like about it is the food report. It gives you a pretty picture in the form of a pie chart which breaks out proteins, fats, and carbs for the week. It's a good way to see what you need to change.
The other stuff on the premium site are more for comparisons to the "average" fitbit user - so they don't provide anything really specific to push you into anything different. I supposed if you were less than average it could motivate you - but otherwise I don't find a lot of value in them.