I love my Stromung, and the Scoobysport has a nice sound t it, too, kind of like the Stromungs, just louder. And those are the two that show the highest gains that I know of. Plust, the dual-tip Stomung looks stock, so it doesn't draw unwanted attention.
I have had 4 exhausts and I'll rank them in order:
1) Kakimoto N1 - top quality craftmanship, amazing welds, a little loud but its expected from a racing-type exhaust;
2) Tanabe Racing Medalion - canister looks awesome, sounds good (a little loud);
3/4) 5Zigen Fireball & Brullen DTM- not very happy with either system. Absolutely zero gains with the Brullen. Craftmanship on the 5Zigen is crappy at best.
Haven't heard many good things about the Stromung system except for the sound. Its ability to rust out within a few months amazes thouse lucky enough to purchase them though.