Spam Filter on Microsoft Exchange

There is a customer who is in need of a spam filter on their Microsoft Exchange 5.5 server. We have checked on the web and found a few and read the promotion material for them. What I would like to learn from everyone is, What spam filter are you using on your Exchange server? Do you like it or dis-like it? What are some good features and bad features? Is it easy to use and administer?

Since I have not used such products, we would like input from others who know first hand usage and information about them.

GroupShield 5 installed for virus protection.