ive just bought the stillen y pipe, did loads of research and heard nothing but good reviews, apart from that it pulls some exhausts out of alignment and causes them to catch but a bit of proper fitting and moving around can solve this most of the time.
The flexi subject is a mixed one to be honest, but if your buying a good exhaust I don't see how removing them would cause it to break at another part, the stillen being wider piping puts less strain on the system anyway as they air can flow more freely.