I have and love my iPad 2, since last April. My data plan is with Verizon, $20/month (grandfathered) for 1GB; in a normal month I use about half of that. I pretty much carry my iPad with me everywhere in a tote bag where it fits nicely.
My non-Apple cell phone plan runs out next week. I would really like to upgrade to an iPhone. However, I would have to get a 2GB data plan for the iPhone from Verizon at $30/month. This in addition to the iPad data plan. I expect to use very little data on the iPhone, since I get my data via the iPad. With my old phone, I have a minimal Data plan for $2/month of which I use very little. I will have to think this over some more...really don't want to pay $50/month for the two devices together. Maybe they will eventually allow you to have a combined data plan if you own both devices.....
Sent from my iPad 2 using iPF