My wife got a WiFi only model and I had pre-ordered a 3G one for myself. After using hers, I still definitely want to get the 3G.
There have been a couple instances already where 3G would have been nice...once when we went to her mom's house, who has no WiFi and the other in the car once where she was trying to access something while I was driving.
Now I did just jailbreak her 3G iPhone and installed MyWi, which works great...but I don't like being stuck not being able to upgrade the firmware (I usually like to have the latest firmware) and two, I don't like that you have to go in an turn on the MyWi and then go into the iPad to connect it to. I want it to just work when I access the internet without having to worry about extra steps.