I use the GPS all the time - my phone is a Blackberry Curve, company provided - while commuting an hour each way every day. With no other way of maintaining a connection the whole time, 3G made the most sense. Also, as I take trips quite a bit, and go to conventions, 3G has been a lifesaver.
I am going to be home most of December, so I will probably not enable 3G and save the money that month. The fact that it is available with no contract is probably the #1 reason I went ahead and got the 3G.