I have used the iPad 2 and iPhone 4 for some little while in the cockpit at various times for various reasons. I have also used both in locations where the GPS is not supported by a mobile connection or WiFi. FWIW I have come to the conclusion, as others have said, that the GPS is not reliable and not fit for purpose if you want / need to be assured of continued reception - it has nothing to do with the app that you are running at the time. That doesnt mean to say that both arent useful devices and nor does it mean that the GPS service doesnt work, simply that it is not as reliable as a dedicated GPS.
It would seem there are two GPSs that are compatible with the iPad - it would be interesting to hear from anyone that has used these - in theory they should enable uninterupted service.