I just got one and posted about it in another thread.
http://motorbicycling.com/showthread.php?t=40137
I like it, it's very quiet with a small "real expansion chamber" bang when you rev it up.
Take off is about 10% better, but to be honest it might be wishful thinking.
It looks great though.
It fits better than my stock exhaust my left peddle would scrape my old exhaust, now there is plenty of room. We did grind and chamfer the flange because there was a huge burr blocking the opening in the pipe. Inspired me to get a better inlet manifold maybe a reed valve next (it never stops does it?)
I like it, it looks good, sounds better without disturbing the neighbors, and there is some small performance increase. It won't turn you bike into a screamer but I think it's worth it.
My boss who has years of experience with 2 stroke motor racing in go carts. Looked at it and shrugged and said it "wasn't big enough" he showed the ones they used and they were much bigger and had aggressive banana shapes to them.