I've been using Mail.app for OS X for a while now. I switched when Jaguar came out because I was curious about its spam filtering. For the most part it works amazingly. One thing I want though is a bit more control. I want to be able to have a white list of people that are by default not spammers. I do have a number of newsletters from companies like CDJapan which it seems to really like to think of as spam. And, I'm afraid marking those as not spam will let through other stuff that is.