I ran across this interesting press release the other day. It's from McAfee, talking about how their Secure Internet Gateway appliance compares to a similarly-positioned device from Barracuda.
When comparing the two devices for spam filtering over a two week period, the Barracuda Spam Firewall failed to correctly tag six times as many spam messages as the McAfee device did.
Find the full report from McAfee here.
(One interesting thing it doesn't touch on is backscatter. See, Barracuda devices send backscatter by default, due to their "accept all mail, then process it" methodology. The big ISPs have been working for years to move away from this methodology. It's not a best practice and hasn't been, for quite a while.)