Enjoying NAN, it's a neat game, plays well and shows off 60fps HD well! We need more games to shoot this high.
However, I don't think it's even in the same league as Super Stardust Delta (Vita). NAN doesn't have the same sense of speed, urgency, or maneuverability. And the weapons in SSD are extremely more fun to fire. Partly their physics, partly their sound, and partly the constant mini upgrades you gain in SSD. NAN feels bland.
The levels in NAN are interesting and beautiful, but often create frustration as much as they add a creative exploration element. Their non spherical shapes often cause blind corners and deaths that can feel cheap. I'm guessing this is one of the reasons NAN feels slow to me: it was probably slowed down to reduce the frustrations caused by non spherical levels.
For me, overall a dissapointment. However, not a bad game. If I had played this before SSD, I'd give NAN an 8-9. With SSD behind me, I'd give NAN a 7.
EDIT: I haven't had a chance to play Co-op. Is it pretty much 2 players playing the same content as a single player? I'd love to try that with the TV and Gamepad instead of standard split screen, could be very cool.