I'm an Avast user, and will never change. But just out of curiosity after all the recent threads, I decided to have another look at Avira intending to return to Avast straight after.
I tried Avira sometime in early '07, but kept getting the same problem. Would not update, and kept telling me the internet connection had failed, which must have been their servers, as my connection was fine and active.
This time, over a three day period, Avira would not update virus definitions at all. Each time it was taking about three minutes to decide the connection had failed.

Avast updates daily, and sometimes twice a day, so my connection is fine.
This has got to be one of Aviras biggest disadvantages, as nothing has changed for me in a year or more, and is the reason I binned it last year.