I am glad that Microsoft came out with the free Microsoft Security Essentials. I think it is a reasonably good antivirus and some people either cannot get an antivirus program, can't afford one, or will not use one unless it is free. The more people who use antivirus software the better for everybody else.
However, I don't think Microsoft Security Essentials is as good as the top paid antivirus programs. In particular, software like Kaspersky, Norton, BitDefender, GData, and Avira.
I do think that Microsoft Security Essentials is probably as good as or better than software like McAfee, Norman, and CA.
So if I was going to use a free antivirus program I would probably use Microsoft Security Essentials. But if a person is willing to pay for quality software, I personally would much prefer Kaspersky or Norton.