I have to say in my limited experience I am very impressed with 3G.
From our boat in the marina (Calcutt) I get similar speeds to what I get from home on an "up to 20Meg" connection. From home to the exchange it is about 3 miles (as the crow flies, longer for the cable run), from boat to the mast about 3.5 miles. I was expecting to struggle to get anything from the marina given the distance to the mast, but far from it. True the 3G is less reliable the wired ADSL, dropping out perhaps once or twice a day compared to ADSL one or twice a month if that. The main difference really is the download limit I guess, but that is just commercial not technical, and can easily change.