I have an iPhone 3GS and an iPad 3G.
I got the 3G model not for the data connection but for the GPS. I use navigation apps both in my car and while flying a small plane (I have a private pilot license). I like location-based apps too.
For car navigation the iPhone is actually enough. I like the visibility of the iPad screen but the iPhone is more convenient to mount in a rental car and to keep at a convenient viewing angle. For the plane, though, there are far more detailed maps with things like radio frequencies and airport names that are much easier to read on the iPad.
I did activate the data plan to try it out, and have not wanted to turn it off since then. I like being connected at all times. I could JB my phone, but I haven't wanted to deal with shorter battery life and other issues I've read about when you JB. I also typically prefer the newest OS features over just about any JB app. That is why I am at the latest firmware level and I don't think there was a JB available until recently. However, there are new OS versions also coming out soon.
If I do decide to JB, it would be only for using MyWi or TetherMe (I haven't looked closely enough to know which one I'd want). For now, one device can back up the other if one battery dies, and I have not concerned myself with the ethical implications of violating terms of service. I can even use my iPhone headset to make VoIP calls from the iPad (mainly using Skype).
I would not want a MiFi kind of device because I don't want to carry and charge yet another device. My iPad, iPhone and laptop are already too much to carry around during frequent business trips.