I have found it very unreliable when flying, while others have reported it to be fine.
I have just found an interesting comment
here
The 3G model of the iPad, in addition to its built-in cellular antenna, also includes an internal GPS. The GPS is completely separate from the 3G data service, so you don’t even have to have an active data plan for the GPS to work. You could buy the 3G model purely for the GPS and never activate the data plan, saving yourself some money. But the on-board iPad GPS was made for ground use, and it’s not very reliable in the air. It has a tendency to drop offline, especially when switching between apps. For this reason, most iPad pilots—even those with a 3G iPad—opt for an external GPS. They are extremely accurate (WAAS-enabled) and very reliable. They also offer free apps that allow you to monitor the GPS’s performance.
which would explain why perhaps some people find theirs OK.
That particular GPS, which seems very hard to buy in the UK (though there are Ebay sellers in the USA charging variously silly postage) also delivers the satellite constellation data, which seems a first for the Iphone/Ipad on which the Apple API blocks that info from apps.
For some reason it
cannot be shipped to the UK so Ebay seems to be the only option. I guess it doesn't comply with the stupid ROHS/REACH regs here.