It's: (1) FAST, substantially faster than Y! mail or HoTMaiL, (2) easy, (3) smart about things like threads and chopping out quoted text (which can be revealed if you want to go back for context), (4) feature-rich--deep search (of course), thread expansion, address auto-expansion, and more, (5) pretty... (ooooohhhh...shiny)...
Anyway, they've given me 6 invite codes to give out (like the "old days" of LiveJournal, eh?)...anyone want one?