I had the MS (Giant) program on my computer until just the other day, when I removed it. It took forever to boot up but seemed to do a decent job otherwise. I never had a false positive occur, nor did their antispyware ever catch anything. I don't frequent gaming sites, children's sites, or porn sites, which might explain my computer's lack of spyware.
Other than the ridiculously slow boot problem, it seemed to function nicely.