I have a Stillen cat back for my 09 Altima. I love how it sounds, and considered the Stillen cat back for the Z, but wasn't a huge fan of how they kept the stock style Y pipe instead of keeping it as two pipes (which I believe the NISMO Z's already have) and I personally don't care for the tips they put on it.. looks too 350Z-esque to me
. I have heard the Stillen cat back on a 370Z though and it does sound nice.
I've got a Fast Intentions exhaust on order