Couldnt agree more, thunderbird is ok, but its fairly slow and hasnt gotten much improvements usability-wise since netscape mail, its basically the same.

Speedwise its not up to par either I think, searching is quite bad, but this might be issues really with imap rather than anything else. My last efforts have been beefing up our mailserver and also adding full text indexing for searching which helped a bit but there is still a long way to go to get it really instant.