I had a similar experience to Michael with McAfee at a previous job. It wasn't finding threats that got in, and it was killing legitimate installs. Eventually IT gave up and allowed people to disable it for 10 minutes at a time to be able to install software properly.

There was a major version update at some point though that resolved a lot of issues and did seem to be working security wise. Can't remember any version details though, but this would have been around mid to late 2009 time wise.