I have used Airnav Pro and don't like it - there's a lot of room for improvement in terms of user-friendliness and speed. It's damn slow, at least on iPad 1.
I agree it can be slow on start, and if you try to scale/scroll the map whilst flying (typically taking 2-3 seconds with my setup, but depends on how much database info you are asking it to overlay), but i still found i don't do this a great deal in flight and that it's very usable, thats why memory map is a good backup as it's less useful but faster as a simple moving map. I too am using the iPad1 and i think on the new iPad 2 it would be less irritating. Still a useful app IMO
iBooks (native apple program) is also good for manuals, plates, lasors, ops manuals etc... and it gives you the advantage of being able to view/arrange your docs in iTunes as well as on the iPad, which you may or may not like...