I've used trend, it seemed to do its job well without interfering too much. Client/Server AV was fine but i found their Exchange/SMTP options a little annoying
Macafee i've never liked, and i hear they have now changed their server licensing to one that is based on the number of the servers users or something, involved in a project at work to remove Macafee from 350 of our client servers and replace with SEP. Even after the time the project will take to replace Macafee its still hugely cheaper than keeping it.. so i hear from the people in charge of the monay
SAV/SEP.. god i hate symantec products, SEP however isnt bad
Forefront, actually a good MS product! >.< my company uses it internally as we get it at no extra cost as part of our enterprise dealings with MS