I have a MBPro 13", 2.66 Ghz wih 4 Gb of ram and a 500 GB Seagate Momentus XT drive. SpamSieve basically locks out Outlook, and slows down the rest of my programs while scanning email.
Why? If I have more than 15-20 new emails when I launch Outlook, it can take several minutes before I regain control of Outlook. Very frustrating.