I had one, very well built. Takes under an hour to fit including removing your old system. Gives the bike more pull right through the rev range( quite a bit around 3.5-4.5k) and really opens up the top end where the bike normally runs out of puff. Oh sounds fantastic too!
Got one fitted to my 1200gs recently. It sounds fantastic and looks great and certainly adds more power at higher revs but seems a bit sluggish below 3000.