As much as I like Watchguard's appliances, I wasn't too impressed with their AV/AS. People seem to love the Barracuda boxes. Personally, I've moved just about all of our clients >20 users over to Messagelabs. Service has been flawless over about a dozen domains, 300+ users. Excellent support if you have any questions, constantly upgrading their network (with no downtime, except the admin panel very rarely). Highly recommended if your company will shell out for a service as opposed to a single expense (although isn't there a recurring charge for Watchguard's AV/AS service?).