+1 for VatScope.
I got an iPad recently so I tested several VATSIM maps, and VatScope stood out from the rest almost immediately. Not only its basic and most useful functions are free but all extra features can be purchased with a very small fee of 3 EUR (autorefresh, self-tracking, VATSIM ID stats, airport weather).
My only gripe with this app is the map itself, which shows only city names but not towns, villages or any other location labels. Obviously there are other non-VATSIM maps that can do that, but I’d rather have everything in one place and in a tablet app.