On the flipside of the other two, I don't really like McAfee in general and have heard of many issues with them. But one client of mine wanted to use theGroupShield product because that is what another consulting group for them recommended. They weren't going to be interested in anything else, so this is the route we went. The customer service wasn't relevant since I dealt with this other consulting group, and then off we went, installing it on Exchange 2003.
The install was "easy" although it took about 7 tries. I couldn't tell if this was a difficult install process, if this server was special, if the person installing it was clueless (I was on site, but not doing the install), or if the product was faulty. After looking back on it - I am going with the fact that the user was clueless (she messed up several other installs that week and so I think she hadn't done them before).
Once installed, configured, the white/black lists are setup correctly, etc - then the product works great. BUT (always the but) the UI is absolutely awful. It runs in Java and is extremely slow (on an 8 processor 3Ghz machine with 8GB of RAM, nothing should be slow) and it is not at all intuitive. It also will toss your settings and other times will warn you that you will lose changes made, but then it won't show you the changes you made - the easiest way around this is to make changes, save, and then exit the app, and then go back in. Suffice it to say, the interface is just extremely painful. On the good side, once you have it setup to your liking, you don't need to go in there that often - especially if you keep your white/black lists in separate text files and as you update them, you can occasionally just go in and reimport those instead of having to use their interface for adding.
I am torn on this product - it works really well in some respects, but it is literally painful to use in other respects.
This is enough for now - this is a long entry and has a lot of pure opinion content in it. I would rather have a few posts of factual references before I veer off on another opinion based rant... for now.