I use paid antivirus at the moment, but having used MSE before, I will never use it again. It has awful detection rates according to tests ran by AV testers and personal experience seems to confirm this. http://www.av-comparatives.org/images/stories/test/ondret/avc_fd_mar2012_intl_en.pdf
Even if MSE happens to be "worst" performer in those tests, I'd hardly call 93.1% detection rate out of 300,000 samples awful. I'd be interested to know what kind of malware MSE has actually missed, whether they actually posed a real threat to the user. Because things like cookie files, registry values, etc are quite benign - something which other anti-virus programs make a huge fuss out of, only to alarm users and spook them to keep paying for a license. Also, Zero false positive triggers in MSE is not something to be scoffed at.
It still missed 21,000 viruses, yes it had no FP's but the others that scored higher than MSE missed something like 4,000 with 2FPs. I'd say the tradeoff is worth it.
Now, as to what I've seen personally on MSE - Personally, an advertisement on a website used a PDF exploit to give me that "MS Antivirus 2010" virus. It killed MSE and infected my computer with ease. I could have easily prevented this by keeping Adobe up to date and disabling the PDF embedding - but I was careless. That being said, what really made me switch A/V was the fact that almost *all* other antiviruses caught (as scanned by virustotal.com) the file that infected me as malware and a few of the computers I had repaired had been infected with similar viruses (they were also running MSE).
I think with the right security procedures MSE would be fine. It just seems like they suck at updating their database with new threats.