So, a couple months ago we switched from an in-house Linux email server to Google Apps for Your Domain. There were some hiccups like migrating legacy email (we paid the $50/user for the premier edition to get email migration support but it was still a pain) but overall it has been great. The most notable change has been the superb spam filtering. Literally, I don’t remember the last time I’ve received a piece of spam since the switch. That’s right, spam is no longer a problem as far as I’m concerned.
I highly recommend Google Apps for Your Domain.