I haven't started training yet, but my recent iPad purchase was partially justified by my intention to use it as an electronic flight bag for charts, backup situational awareness, etc. I've also read a lot of online articles and blogs about the iPad's usefulness in the cockpit.
The best article I've found so far is Greg Brown's Flying Carpet blog:
http://gregbrownflyingcarpet.com/2010/10/14/ipad-for-aviators-yes-the-3g-model-offers-gps-and-no-you-dont-need-a-data-plan-to-navigate-aloft/ He has several links in that article to other sources of information. There is a NASA ASRS report that describes inaccurate GPS fixes when using the internal iPad GPS:
http://www.nafinet.org/APPLE_IPAD_USED_AS_AN_EFB.pdf
Sportys also has a good bit of information about using the iPad in flight on their iPad Pilot News site:
http://ipadpilotnews.com/
The wireless-only version of the iPad doesn't have a GPS built in. You will need one of the 3G/4G models for that. But even if you get a unit with a built in GPS, I would hesitate to rely on it for location data. It wasn't made with aviation in mind, and there are many reports about inaccuracy and gps services not working correctly while using the iPad's internal GPS.
The take-away for me:
- I won't be using iPad's GPS-enabled apps (like ForeFlight) without an external device. External GPS units can be purchased for between $100-$130 (google Bad Elf or Dual Electronics).
- If you are wanting access to more than just GPS location data (Weather, NEXRAD radar, METARs, TAFs, PIREPs, TFRs, etc.) you will need an ADS-B unit (no subscription needed). They will run you up to $799 for a Stratus unit. SkyRadar sells three units ranging in price from $599-$799 (I haven't read much about the SkyRadar unit, so have no opinion on how good/bad they are). These units will provide you all the data that you get via ADS-B, plus the GPS location data. ADS-B transmitters are all ground-based as far as I can tell, so the receivers work best the higher you go. You may or may not have access to the full flow of information while you are on the ground, and different ground transmitters send out different types of information (see this link for more details:
http://ipadpilotnews.com/2012/05/flying-with-stratus-a-practical-perspective/)
There is another, more expensive route, providing XM Satellite access to weather and other data. XM external units sell for quite a bit more than the Stratus ($1000 or so the last time I checked), and also require a subscription service. They don't provide access to the same set of data that the ADS-B units provide (for example, Pireps). Due to the higher price and added cost of the subscription service, I haven't looked into them as an option for me.