I had the original, and upgraded to 3G. Yes, it is as fast as it claims to be (independent test claimed it was the fastest 3G phone when it was released). I certainly notice the difference.
I think the 2G phone was just fine, but I’m happy to have fast, constant Internet access whereever I am. Of course that comes with a higher cost, but so far I’m happy with that.
Probably the biggest difference though is the GPS and Google Maps. I use the transit lookup feature almost every day and it’s really helped me get around. Funny thing is that, before I got the 3G, I never thought I’d use the GPS.
So, I think if you are getting a data plan with the phone, then the 3G is worthwhile. If you are happy to just use wifi, then save some money and stick to the 2G.