It all depends on what your needs are and how you drive it.
For myself, I think the new 400z looks ugly, especially the tail end. It looks like them older models of the 240z. Good job in keeping some 370z look though like the door is straight up from the 370 but I was expecting for more when it comes to the new generation and design.
The engine is of that of the infinity q50. That is the first thing I look for when it comes to new cars because a lot of time, they just borrow other car's engine. Like the acura ilx, overpriced with a civic engine. How about the supra with the bmw z4 engine? This has the q50 300hp engine. Get the nismo and it has the infinity red 400hp engine. Now why don't they throw in a gt-r engine? Pricing would be a tad higher though but could sell like hot cake.
I'm a 4 banger kind of a guy. Well that is because I like my spark plug out front which makes it easier for a tune up. Them 6 cylinders are tough on the last 3 plugs which are way way at the back. I haven't look under the hood of the 400z so I wouldn't know but I'm just saying as in general.
Gas mileage sux on the 400z. In comparison to the supra or the z4, it is not even up to par however, it does has a 20 gallon tank so you are talking about over 500 miles on a full tank so that's a plus.
Overall, if it was up to me, I'll lease me a bmw z4. It has a convertible if flossing is your thing plus with leasing, I can ditch it when I change my mind because I find something else on the market that I like better.