I've been using the new (Jaguar) Mail.app since Sunday. My first impressions were quite negative. My biggest complaint was I could no longer use my SpamAssassin setup because UNIX mailboxes were no longer supported, and Mail's junk filter wasn't as smart as SA.

I'm thrilled to report that Mail's junk filter has improved dramatically! In fact, it's catching more spam than SA was, and tagging way less false positives.

Congrats, Mail.app team!