The 3G is nice when your away, a word to the wise, I can get a solid 8-9 house of non stop 3G use (source: last road trip). With one of those tethering dongles, you have a limited 3 to 4 hour battery life before your offline, unless of course you can plug it in.
I don't use it often, in fact the 3G version was a gift otherwise I might have bought the wifi, but it sure does come in handy, in hindsight, I WOULD have bought the 3G version, if I had been the one to buy it, and I had realized how much it comes in handy, if that makes sense.
You cannot add it on later, remember that. If you can afford it, do it. The plan is a la cart, buy it as you need it. I don't auto subscribe, I just buy it when I need it, if I still have time left great, if not, I buy more. Otherwise I might go for a month of paid service that I didn't really use.
The GPS is also a fantastic feature!