The voice quality is not very different in my opinion, but on 2g, people will get dropouts - a lotta times the volume will shift for them talking to you, or you won't hear them. Also, if you get a push notification or text, it can drop your call very easily - this happened with my iphone 3g until i turned 3G service on.
3G definitely uses a lot more battery, but it's much better than the headache of many dropped calls on 2g.