I have the Traxxas Resonator pipe on my 2.5 and the Integy dual pipe on my 3.3, I love them both but the dual pipe is very loud. The Traxxas resonator pipe does have it's benefits, and it sounds really mellow.
I got my resonator pipe free, so far it was my best add on to the 2.5 (cost aside). I added that, and a motor saver filter. After a re-tune this thing really woke up. I'm very happy with it, but again it's the only pipe I've used on the 2.5. The integy pipe, to be honest the rear exit is nice because it doesn't make the mess on the body. The way mine is mounted it is protected by the bumper and the wheelie bar. If you were to hit something from the rear really hard I would immagine it's going to break the header or the pipe or at least bend the crap out of it. That is really the only downside to it other than sound, my 3.3 is heavy and still moves very fast with the pipe. I will say it's harder to block both holes to prime the engine or to shut it off though.