I have the WIfi only and I have to admit that lately I have been thinking that if I were to buy the iPad two I would get one with 3G also. It would save me having to carry around the MiFi unit that I have. The MiFi is pretty useful though and does the job well. I can connect up to five devices to it and that is a neat trick.
The main thing with the MiFi though is the battery and how long it lasts. If I had the 3G connection in the iPad it would be very useful in that I could get a longer session of being online without having to worry about how many minutes I have left to use the battery.
I was able to get the MiFi from England from the phone company called Three, there were the only ones that were selling in such a way that it was affordable to get and unlock for use here in Spain. I have it set up with MasMovil and it is working out very cheap to use. I just looked at my account and I see that I paid €15 and have it set up to renew automatically as a PayAsYouGo account. I have only paid the one amount and never had to top it up. Just goes to show how much I use the iPad on a 3G connection. Maybe it would be a waste of money to get an iPad with the 3G built in?