For MSE being free, it's the best free AV client I have personally tested and I've tested them all.
Personally I use a paid AV client that offers better protection. However MSE is a great choice if you are planning on using best surfing practices and not lurking around in the underbelly of the internet.
I have found a few things I don't like with MSE while testing. 1. Seems when there is an issue with MSE you really can't get support to resolve it. Best to remove it and all settings then re-load fresh. 2. Malware that is self installed often can slip by and seems to be able to disable MSE easier then most paid clients on the market.
Just my personal experiences with it.