Well, you will find two proponents of prepaid plans here - myself and bdn39. We're in the MA U.Plan and bdn39 is in the PA plan.
Before signing up for the Independent 529, you should search out all of the prepaid plans which are available to you (dependent on what state you live in).
I personally do not like the Independent 529. All of the schools in the plan are private schools, are all expensive, and most are second and third tier schools you've never heard of. Yes, you have the handful of Ivy schools and other top/well-known schools. However, most kids will not attend the Ivy school and so other schools would be nice - some kind of spectrum of private/state schools. The conclusion I came to after some time is that most of the schools in the Independent 529 are members for marketing purposes - they do it to get at a wider variety of applicants that they never would have previously. Because they are in the program, and students in the plan will most likely attend one of the schools in the plan, they have a much higher probability of getting more applicants than they did in the past.
In any case, in general, most people are attracted to prepaid plans for the exact reason Drew indicated - you make your payment and then you are guaranteed what you will get in return. Unlike a 529 savings plan where what you ultimately receive is entirely dictated by the stock market.