It's a graphical achievement, and it's a launch title no less. Sure, the plot is paper thin and the campaign is short at 5-6 hours, but for the price and the console, you'll be hard pressed to find anything better.
If you're a first person shooter fan, and you have an iPad, and you care much more about blasting mindless aliens with a shotgun than being immersed in a riveting story, then Nova could well be for you.
N.O.V.A. is still the best shooter for your iPhone, so I recommend you definitely hang with that version. N.O.V.A. HD is just too clunky to get behind, even though it has all the same great set pieces from the iPhone hit.