As a private pilot, I've been using an iPod Touch to run apps for flying starting in 2009. I have been extremely pleased with the performance and functionality of the iPod Touch, but for me there was only one problem; the display is too small. As soon as I heard about iPad, I knew that I could not wait to get one just to run ForeFlight. I pre-ordered a 16GB WiFi Apple iPad which arrived April 7, 2010. I've been using the iPad now for a couple months and have found it to function as promised (in fact, just like a big iPod Touch.)
ForeFlight Mobile 3 HD is the most comprehensive aviation app available right now. Get aviation weather, AF/D & flight plan filing with maps, TFRs, TAFs and METARs. At $74.99/year (the most expensive app I have purchased)... it is well worth it.
You can save AF/D and plates/procedures based on location and ForeFlight will notify you when they have expired. This is an important for me since I have the WiFi version of the iPad which can only be updated when connected to a WiFi internet connection.
With the launch of the iPad 3G, you can now access real-time data when connected to AT&T 3G data network. I think this would be very helpful for sitting in the plane to check weather, otherwise ForeFlight will let you download entire states to your iPad in the event you cannot connect.
For only $2.99, AvCharts is a newer app that will let you save US approach plates, airport diagrams, STARs, and SIDs. AvCharts even has a "draw mode" so you can take notes on charts. AvCharts claims to update 1 day before their expiration.