Long time AVAST user. Generally have not had any issues aside from specific cases (Punkbuster and BF3 conflicts).
With that said, Windows 8 + Latest version of AVAST is a mess. I have two W8 systems, one has the well reported and common blue screen of death issue.
Both systems experience issues such as, instability with CHROME(webrepp plugin not installed), causes chrome and other browsers to want to save a webpage instead of going to the URL, slow down in browsing etc.
When AVAST is removed and cleaned with the uninstaller the systems do not experience any issues.
Now I rather not switch to another AV, as I use and like AVAST for ages. Is an update coming soon?