I have a 3G 16 iPad and I'm really glad I opted to go with the 3G. I think that having good wireless data anywhere I go is one of the main reasons I like the device as well as I do. Where I live, the 3G coverage is great. Even at my farm out in the country I have really good reception and my speeds on the speed test are showing excellent results.
I could see it being an option some people wouldn't need, but for me, its invaluable. I use mine for instant messaging a good bit as well as email, and its quite convenient to grab it and go without worrying about where there is and isn't WiFi. I can tether with my phone, but its so convenient having the service built in, especially since it doesn't require a contract anyway, if I decided to cancel I could do so.