Microsoft Exchange's Intelligent Message Filter blows big meaty chunks. I'm sure it's bright enough, but it has essentially no management interface. Just a couple of registry hacks. And no site-wide whitelisting! WTF?
On the other hand, Symantec's Mail Security for Exchange (SMSE) seems damned nice indeed. I should've installed it much sooner, but other projects kept getting in the way. Not now. User-defined regexp filters! Woot!
However, I still [heart] MailScanner + ClamAV + SpamAssassin. With this combo, I was doing everything SMSE does four, five years ago, for free. In contrast, SMSE cost us north of eight grand. MailScanner in particular is probably the best open source project I've encountered to-date.