I agree... I just got this phone and using on T-Mobile unlimited plan... Its an awesome phone... And the build quality is nice... Feels good in the hand and would definitely recommend it...
I just pulled the trigger on getting an N4, but I had the same concern before doing so. What I figure is that if the N5 is a huge upgrade, I can sell my N4 and buy it. I reckon that's the beauty of non-subsidizes phones without contracts.