I've been using Limewire for three years now and never got a single virus from it.
The thing is, the actual software doesn't contain viruses... But because it's peer 2 peer the files being put on it can have them.
I generally find you're on the safe side if you ONLY download music with it. With music, it's easy to tell if a file's legitimate or simply a virus. Don't download applications like, ever... Even if an exe file looks legit, it can still have a virus on it.