Fantastic review, Innuendo. I've been using AntispamSniper for over a year now, and it definitely generates less false positives and false negatives than PopFile, which I had used before - and PopFile was already pretty good. The toolbar is nice to have, and the configuration really needs very little maintenance.
Without ever tweaking the weighting, AntispamSniper gets almost everything right. Sometimes it's a bit hard to teach it very short messages (one-liners) as spam, and it takes a few of those before AntispamSniper learns to recognize them - but like I said, I've never bothered to tweak the filtering options.
My favorite feature of the shareware version is the automatic whitelisting of recipients in outgoing mail, really handy. Other than that, the freeware edition is probably fine for most uses.