Pros:
- You can define custom actions to run, including running external programs
- Very keyboard-accessible (including bindings for those custom actions)
- You can define boolean filters to process messages as they come in, but also per-folder processing rules to run when you visit a folder. Again, you can run external programs as one of the actions when a filter or processing rule matches.
- You can write whatever From address you want. A lot of mail clients make it way too hard to do this, but it's important if you have multiple aliases or a catchall at your domain.
Cons:
- Seems to be single-threaded. Certain UI and network operations block the rest of the UI.
- Has a good search feature (same as filter rule language) but not great -- it doesn't build an index and doesn't search across more than one folder at a time.
- Can't send HTML email (whatever) and can't view HTML without a pretty clunky plugin (annoying). I usually use a keybinding to send HTML emails to a Firefox instance that has networking disabled. :-P
- One big keyboard binding that's missing: You can quickly get a list of all URLs that are in the email, and select and launch one, but not just copy it. (Most people won't care about this, but I prefer opening some email links in a private browsing window.)