I've tried it. Seems to have a low system footprint and works as it should. I've never had a virus infection that it's had to clean off, but I'd say its as good a security blanket as any other program.
Pretty much echoing what the two others have posted above me have said. While I've never had a virus with it yet, I don't mind having it around due to its low system usage. They have a 30 day trial, so you can give it a test drive if you want to see it in action.