I have a Magnaflow I bought from ebay. It is stainless, but it is not a high grade stainless so I expect that it will eventually rust too. The tubes are 419 stainless and so is the welding wire that I used, so it will probably outlast the muffler, but I'm not too concerned.
I do love the way these sound. I will say that around 2500 rpm it does resonate in the cabin and it is loud, but above 3K rpm's it quiets down and sounds good. Driving through my neighborhood I can set off car alarms with it