My main issues are with Mail.app. It has several issues, especially with IMAP (at least on my machine and email servers of choice). I've reported the bugs to Apple, so hopefully the fixes appear this time.
Sweet more updates! Man Apple has really been on the ball lately. They completely changed the internals of quickitme to improve safety. They have fixed many problems in safari, including the one that brought down the MB Air at PWN2OWN. Seems like a short time ago they released 10.5.2 which fixed a lot of my problems. GO Apple. Keep the updates and safety coming.