This is one of the most contentious issues debated here with very intrenched positions on both sides.
There are those who will swear by AVG and those who say it can't be as good as purchased AV's.
It is often stated that free AV, Anti Spyware and Firewall and a computer used by someone using sensible precautions is all that is needed. Yes that is probably true if you have a user/users who can be totally trusted to act sensibly.
However if you have a machine used by those who know little about computers, students, teenagers etc then having the security of an all in one package that updates itself, can be compelling.
I have two machines running Avast, one running AVG, one running Norton and each has Windows Defender and are all behind a well configured hardware firewall.