iPad for in-flight nav
My collection of thoughts on this topic based on a few months of experience:
1) iPad 2 not stable enough to be considered as a truly solid piece of avionics kit. Irrespective of the software package (I've tested SkyDemon, AirNavPro, MemoryMap), the iPad seems to crash (requiring a re-start of the app or even a reboot of the device) a few percent of the times I use it.
2) Because of 1), I use my trusty Garmin 430 as primary GPS device (fair enough -- it is IFR approved, and I fly quite a bit in IMC). I use the iPad as a convenient source of useful flight information (via 3G) -- mostly when on the ground, pre-flight, and as a fun backup "toy" in-flight (i.e., not ever relying on it -- except if my aircraft were ever to suffer a total electrical power failure).
3) An external receiver (I find the GNS unit to be highly reliable -- just remember to keep charging it between flights) is a "must" if using the iPad as a GPS moving-map in flight. The built-in GPS is effectively useless in flight (failed due to lost satellites on every route I have tried it on. By contrast, the GNS has only lost satellites a handful of times).
4) The iPad is essentially too big for my cockpit (Scottish Aviation Bulldog). With a stick (rather than a yoke), there is simply nowhere to put it (mounted on the knee, it gets in the way of the stick, anywhere else forward of my seat, it blocks too many instruments or my ability to see out the windows). I therefore tend to jam it between the seats, and only get it out for a look every so often.
5) All in all, the best use I've found for the iPad in flight is for reading the approach plates from the Jeppesen TC iPad app. This app crashes, too, but quite rarely. It is a great solution for keeping plates up-to-date (no need to spend ages each month stuffing paper amendments into the binder etc...I know, I know I'm obliged to carry the fully up-to-date paper plates on-board when IFR....).
In summary, the iPad is a fun accessory with slick apps. But don't over-rely on it in-flight...