MSE recently lost its certification from AV-Test, which (obviously) isn't good. Also, I've found it to be less able to protect systems than other free AV software. I often help friends and family with their computers, and I've had to clean way too many systems that were supposedly protected by an up-to-date copy of MSE. In fact, all but one of the last 10 or so were MSE, the other was Norton I think. Anyway, some were quickly reinfected until I started replacing MSE with Avast. Since then, I've not had to re-clean any. So for me, MSE isn't worth the price you pay for it.
Just a Shooter (and professional computer geek)