Just like everything else, including ADSL or cable etc, it is dependant on your location. You should really check that you can get a decent 3G signal in your area before ordering to be honest.
3G is good where I live and although I'd never use it as my main connection I do use it as a backup when my main net goes down or I decide to play with my router etc. It's pretty fast, get a good 500kb/s download from it usually and pings of ~120ms to UK sites.