We got an ipad2 with 3G four years ago and have an AT&T domestic plan, $25 for 2GB every 30 days. It also can be used alternately with wifi when you have a good signal by just changing the setting (5 seconds.)
I believe the 3G tablet to be one of the most useful inventions of my lifetime. I use it throughout every day.
Streaming movies or TV gobbles up a lot of data fast, however, so we don't use the 3G service that way. Maybe if you had unlimited data and a strong signal...
Our phones are el cheapos that are only for making calls.
Aside from the unreliability of campground wifi, the 3G ipad is great in an RV for the copilot to use surfing the net even while riding down the highway to check email, maps, Gas Buddy prices nearby, find campgrounds etc. Yes, there are some dead zones, but fewer as the years go by.