According to the Mobile Operators Association, if our mobile phone companies are going to operate 3G networks reliably, they are going to have to massively increase the number of masts they have (possibly two or three times the number they have now).
Basically the problem is because the protocol used by 3G transmissions needs a higher frequency, each Cell has to be smaller. Smaller cells mean more masts need to be installed to cover the same area. The technology compounds the problem by reducing the size of cells as the number of users go up.
This might explain the problems experience by various 3 users I know (well, apart from the cr*p customer service). 3 appearing to have problems would certainly be explained if they did not have enough masts.
Full story:
http://www.theregister.co.uk/2005/04/15/3g_mast_wars/
The advantage I can see with going for 3G is that data transfer will be faster. I'm just not sure I want triple the number of masts installed to do it.