I went the over-install route from 4.8 to 5 (free versions in both cases), and have had no system problems on my XP-SP3 (meaning primarily no BSODs, yet anyway). One overall nuisance was that I had to reboot 2 or 3 times after the installation before some features (like for example the scan tab) became fully functional. And it took a fair bit of playing around to get auto-updates to work properly -- thanks to David for noting that ticking the "dialup only" option seems to disable update checks regardless of your type of connection, unticking both choices fixed that beautifully.
"Pause provider" seemed to be missing, but you can now opt to "stop" a provider for a predetermined length of time, e.g. 10 minutes or 1 hour, and not even have to remember to re-enable it. Or, of course, if you finish whatever else that required pausing it (in my case, moving large numbers of files) earlier, you can always restart it at any time.
On my system, at least, the only thing I can find that's not working is auto-updates to the program itself. And I think it's been made pretty clear by the Alwil team that that's not a malfunction, they just haven't finished working on that yet.
There's only one feature of the former versions dropped from v5 that I miss, and of course if I can't see it for looking I'd be delighted to have someone steer me to it. That's the warning logs/journals/whatever that were internal to avast. Oh, and another one I just remembered -- that "click for details" thing on the auto-update notification popup.