Pros: Same great Nextel service - just no contract :)
Cons: Older internet (Hey the iDEN system is still TDMA). Text messaging is via the web.
Summary: I bought the BoostMobile Starter Pack ($19.99+tax) and am using my i560. Service for the first few hours after activation was kind of flakey - to be expected - but afterwards, awesome. The same great Nextel service I was used to and the CS reps ARE friendly and based in the US.
Some people who post here are just idiots and remember, for every BAD post you read - there are at least equally if not more satisfied customers.