I just installed my GHL catback today. I'm also impressed with the fit and finish of the tubing and the mufflers. The clamps, on the other hand, are not very impressive. The stainless bolts they come with are way too soft to be able to tighten properly. The threads gall before the clamp is even close to tight. I had to cut the bolts in half with a Dremel tool just to get them back apart. I replaced the bolts with 1 3/4" grade 5 3/8"-16 bolts which worked much better, but the clamps still do not seal the tubing well at all. I'll probably pick up some stainless lap-joint band clamps like these:
http://www.summitracing.com/parts/WLK-33226/ and replace the ones that came with the system. Still a great deal for this system compared to many others on the market.