I know this forum is about members reporting problems, but as some folks have pointed out, there are certainly many more satisfied users than not, but the satisfied ones don't post.
So let me say that I upgraded from 5.0.677 to 5.1.864 on 2 machines last night and everything went about as smooth as anyone could ever hope for!
avast! keeps getting better, and my confidence grows greater every time another upgrade is released and it all goes so well.

(Edit: version # correction)