I have the iPad2, 64 gig, verizon 3G. Pay $20/mo for data (1 gig). I use the data plan for email, websurfing and checking my schedule and the weather out on the ramp. Occassionally send/update jepp charts on it via cellular too. Have yet to go beyond my data limit, but I also don't do a lot of streaming music or video (maybe 30 min/mo).

Charts (NOAA and Jepp), local sectionals/enroute charts for weekend flying, and all of the manuals that I could think to put on (aicraft, FAA, company), plus some fun apps/games, photos, and music and I still have over 35 gig of free space.