We’ve also used Vipre, and most recently, Sophos. We converted to ThirtySeven4 around a year ago, and we’ve been really happy with it. It’s MUCH cheaper than either of the other 2, and seems to pretty much just do its job invisibly, which I really like. Central management is simple to use and effective. The people have been great to work with.
Anyway, I highly recommend it! Don’t really see any benefits to any of the other companies, and the price can’t be beat.