I'm guessing from the relatively few replies that the answer to many original question is not many;
There were at least two recent threads about the iPad on here. I have linked to one of them earlier on. There's not a lot that happened in the iPad world inbetween those threads and this one so I'm guessing a lot of people choose not to duplicate their posts.
Anyway, yes, the iPad has its limits and I'm pretty sure there are better devices to be found if you need a dedicated navigation tool. What appeals to me however is the versatility of the device. To a large extent it's the only electronic device I take with me on flying trips, holidays, work, hobby and whatnot. I use it for flight planning, flight execution, watching videos, listening to music, sketching, taking pictures, Geocaching, playing games, reading books, doing presentations (with an iPad<->VGA cable), annotating PDFs I have to review, photo album, browsing the web, twitter, catching up with the news, planning car and public transport journeys and so on and so forth.