This is a post on the Gmail blog about their spam filter. The two biggest features Gmail offers for personal e-mail and hosted domain e-mail are the spam filter, super-fast searching, and very large mailboxes. Oh... The three biggests features are the spam filter, super-fast searching, and very large mailboxes... and a great calendar. Four! The four biggest features... Oh well, you get the idea.
Official Gmail Blog: How our spam filter works
We previously hosted our clients e-mail on one of our servers, but ever since the Google Hosted Services / Google Apps stuff went public, we've been moving our clients over to use that instead. We've been using it for our sypher.com domain e-mail since it was in Beta, and I don't think we'd ever move back to hosting our own again. I'd highly recommend this for any organization with less than 25 e-mail accounts, and probably even for you bigger companies out there.