I've used Y!Mail in the past, and been pleased overall. However, while their filters do catch about 95% of the Spam, I had problems with false positives. I, personally, would rather see a bit more pink meat in my mailbox and be assured that false positives would be very rare.

I've been pleased with GMail in this regard, but for real "industrial" use, I use Spam Assassin with a whitelist and a threshold score of 6.5. That seems to nab about 80-85% of the Spam, and I have had 0 false positives in about 6 months.