Owned 3G models of both iPads. I think their usefulness is very much situational.
a. Do you need or want to use your iPad in places where wifi is inconvenient or nonexistent?
b. Do you mind the $15/month
c. How often do you need the 3G access?
At first, I found it tremendously useful. My job at the time did not have an employee-accessible wireless network, so the iPad 3G gave me a great way to seperate core work from browsing/email/research. It's really damn convenient to just go sit in reception or the lounge or the coffee shop down the street and browse the web with no worries about wifi passwords, signal strength, passthroughs and so on.
Now though, I can't justify paying for 3G access.
1. Rogers gives free iPhone tethering on certain plans
2. I work at a business with employee wifi
3. Most of my breaktime requires a Macbook, not an iPad
So basically, it was very useful for me but I don't think I ever reached the amount of value needed to add that feature to the iPad in the first place. With iPad 3 it's very likely I will go wifi only and just put the money towards storage instead.