For what it's worth, I don't think I've ever seen MSE actually stop any kind of virus, trojan, or piece of malware from installing and running. Ever. A few weeks ago a family member clicked on an e-mail attachment which then proceeded to install some feux AV scam which made the computer unusable while constantly throwing up scare notices about porn viruses (the scam was they wanted you to send them your credit card info so they could send you a "key" to enable their software and remove the "virus".
Their PC had a fully up to date MSE running which proved to be utterly impotent. I had to run RKILL to find and kill the process before installing and running MalwareBytes as well as HijackThis and another program I can't remember the name of so I could track down and delete the remnants. While doing this I had the process manager open and found that "iexplorer.exe" was running, even though Internet Explorer wasn't. I killed it and a minute later it came back. So it had another trojan on it as well, apparently, that MSE wasn't aware of.
By far, the best anti-virus is common sense. But if, on the other hand, you have family members who think browser toolbars are obviously an essential part of the Internet experience (and don't we all), then you have my sympathy. I can only recommend that you also make sure you have programs like MalwareBytes and Spybot S&D installed to help catch what MSE can't.
Candidly while I don't think its true that MSE won't stop *anything*, it will stop less than the likes of McAfee, Norton, and Kaspersky. MSE does not have the same newer detection technology that the big guys have and does not innovate at the same speed. But it's free and is often good enough for a lot of folks.
Put it this way, the free stuff will block say 90%. The good pay stuff will block another... 8-9% on a good day. There's >1-2% no one will block. Whether that 8-9% is worth the $50 or so a year is up to the individual.
*disclaimer* Once again I do work for one of the major pay security organizations noted above and my opinions are my own, not theirs.