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.
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.
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.