I've been running MS Security Essentials for the past few months and I've had no problems at all with it. As far as I can see it's working just fine. I've been running Webroot Spy Sweeper with it's antivirus to check and see how MS SE is doing, and so far everything is ok. Neither one has let anything in. My Spy Sweeper subscription runs out in June and I'm not going to renew it.... I'll tuff it out with MS SE only from now on.
So I say go for it, it'll save you money.