It depends a little bit what you want the app to do. Do you want it to teach you about space, or do you want it more as a reference with a large star/object database for when you're out observing?
I started with Starmap Pro on my iPhone to use as a reference, for which it's very good. Here's the
iPad version. Later, I picked up
SkySafari 3, a universal app, which is also very good, and much cheaper (much smaller database).
I haven't looked at this much yet, but it's free, and seems to have some good reviews:
GoSkyWatch Planetarium. I think this is less a reference, and more for education(?).
The best, though, was Planetarium for the Palm Pilot. (Anyone? C'mon, someone has to remember!)
Regards,
Tom
p.s. I'm not an expert, but I play one on the internet.