i dont use 3G at all, even though i have the 3GS
3G signal tends to be weak where i live and the areas i often go to - this means the radio has to work harder, thus less battery life.
fine it takes a few seconds longer when im standing on the platform trying to look up the departure boards for the next train - but you get used to it.
and generally when you need to use your iphone for proper internet use, you have wifi.