One problem with a totally self contained GPS is that it may not get a good reception in a metal cockpit.
The Garmin x96 units are pretty good but not perfect, in this respect.
And user reports on GPS-capable "gadgets" are highly variable. For example I know of owners of Iphones who say the GPS is crap, and some say it is great. Same for the Ipad; some find it great and some find it crap.
A fairly persistent thread, reference the Ipad too, seems to be an inability to acquire a lock if switched ON while already moving. This is a very old issue with GPS receivers. I have a Fuji LOOX N560 PDA, a Thuraya/Hughes 7100 satphone, had a Thuraya SG2520 satphone, and none of them will acquire a lock while flying. The N560 (which I use mostly to run TomTom) won't even acquire in a car if driving... well it does after maybe half an hour.
So I would caution against consumer IT hardware for this.