350Z is a great car. I would say the interior is more plain than it is cheap. Keep in mind that the ride really is very harsh. My G35C has the same suspension setup as the 350Z except for softer springs, and the ride is still really harsh with my car. The 18" wheels hit the bumps REALLY hard but I have never had any problems with potholes and stuff, and I take my car into NYC almost every week-end.
You should really test-drive one before making the final decision, unless you have already. The engine is really great, lots of power, smooth power delivery, and it SCREAMS at high rpm's...an almost truck-like tone to it, I dunno if you like it or not.
The transmission is really great too...shifts are notchy, but the gearbox is very precise and shifts are so damn close and cleaer, every other manual tranny feels like a step down.
If it helps you out, I can list the things I hate about my car....I know it's not a 350Z, but they're pretty similar cars.