For those that are saying 70% or most of the infected computers they clean have McAfee or Norton installed. Did you ever stop and think that is because they have a large chuck of the security market? Plus, if someone is hell bent on installing a "free screensaver" or "free smilies" or surfing Pron they are going to do it despite the warnings.
McAfee, Norton or whatever cannot save a person from their own actions. Security is a process and not something you buy in a box at Wal-Mart. IMO, If a person uses common sense, keeps Windows and their programs updated and uses McAfee's suite, I think they'll be fine.
P.S. I just checked av-comparatives.org last main test which was on-demand only and was in Feb. 2009 and McAfee AV scored Advanced+. Another test which is Proactive should be coming out in May so let's hope they keep up the trend.