A long time AVG user here, I abandoned their products shortly after the release of version 8.0. Their redesign was nice, but resource usage shot through the roof while detection rates plummeted through the floor. Unfortunately, it's remained this way ever since. Their antivirus and internet security products are both huge resource hogs, which when compared to other suites, make the price and impact unjustifiable for running on any of my systems - what I mean is there's other, full-featured suites which pack a lot more into a package with a much smaller footprint on system resources.
Believe me, I am an avid ex-AVG fan and still beta test their new products, in hope that they have improved vastly. Just last week I tested AVG 2013, and am disappointed to say it still eats resources and performs like a fat man eating donuts - sluggish, slow, and arguably inefficiently.
avast! on the other hand - lighter system impact, and I admire it articulating to me when the virus database has been updated.
However, I stick with MSE all the time now. It's got practically no impact on performance and great detection rates. With it now integrated with Windows 8, I really don't ever see the need for another antivirus program. I may still use COMODO Firewall in the future though.