Exhausts are such a personal thing it's really hard to describe. Put it this way: I hated the sound of the stock 370 exhaust and I loved the Stillen I replaced it with, which some people regard as a fairly quiet system.
The Nismo exhaust is definitely louder than stock 370 and probably reasonably close to the Stillen - maybe a fraction quieter. If you don't want a really loud system you might be happy with it. If you're more towards the Fast Intentions exhaust with high flow cats end of the spectrum, you will find the Nismo too quiet.
I would prefer it to be a bit louder but I can live with the volume it puts out. I don't feel the need to rush out and buy a new system and I'm not slightly embarrassed by it like I was when I returned my previous Z to a stock exhaust for the sale. The Nismo sounds like a sports car, whereas I personally think the stock 370 system sounds like a normal family car!
No idea where you live but if it's anywhere near me (Twickenham, Greater London) you're welcome to come over for a listen.
Whatever happens, if you don't like it you can always change it.