PRO's:
Free, competitive detection rates (90%+, even 98%+ in some tests for malware/adware/spyware), very few false positives, usually low system utilization, easy to use, great reviews for a MS product...

CON's:
Lack of configuration options, reportedly slow scanning times, no "pause" mid-scan, no firewall (Win 7 firewall can be great however).


I personally use it on a few machines at home and I like it. That being said, I am a big Microsoft advocate as of the past year or two and I think they have done a lot to focus and change their image re: security. Just like most AV software, it's not a complete solution and it's not going to detect everything when you throw a stupid user at it.

Also consider that it is a stage 1 product (if you don't consider it to be an extension from OneCare). Microsoft will likely continue to refine the product as they go and it will get even better than it is. It is in Microsoft's interest to minimize Malware/Spyware/etc.

Thats my two cents.