I get very little spam that isn't caught by Gmail (personal) or Zimbra (work) spam filtering setups. Not even an average of one per day in each account, I'd say. When one does slip by I always mark it spam to train the system (this is something I wish I could get more users at work to understand). I haven't had any problem with false positives. Seems to be working very well. I usually don't even look in the spam folders anymore.

My Gmail spam folder has about 1200 items, and at work the folder has 150. They auto delete after 30 days, so that's representative of what I get per month. I think the work number is so much lower because above a certain threshold that server actually kills the messages before they even reach my spam folder. Not sure that Gmail does that.