We started using SpamTitan recently. I have evaluated a number of anti-spam options and many of them either had difficult interfaces, or were painfully slow. Setup of SpamTitan is very simple, and I find that 99.9% of spam is being blocked. The few emails that do get through, I am able to teach the product about them via SpamTitan's outlook plugin. SpamTitan doesn't use very many resources, and mail is processed quickly.