I purchased the 3G data plan. I'm usually in wifi areas, but I take my iPad with me everywhere. It's definitely worth 30 bucks a month to grab Lily (my girl's daughter named the iPad) and get in the truck and have access to all the features. If I'm shopping, I'll grab it and look up reviews to consider a purchase, or if I hear something on the radio I'll jump on iTunes and download the song or album before I forget.
Personally for me, the big appeal of the iPad is that it's so mobile, just grab and go. Having to be in a wifi location would take away from a lot of that convenience. Also, my girl and I chat throughout the day on AIM and I stay signed in to get the push notifications.