I really like the new Mail.app in Leopard. With Gmail moving to IMAP, I was finally able to get virtually all of the functionality that I'm used to in Gmail on my desktop Mail application, which is really cool.
Well... except for one little thing. Mail.app has threads, but it does not thread Sent Mail in with Received mail. In Gmail, on the other hand, inbound email gets threaded with outbound email, so I get a nice breakdown of the actual discussions I've been having. Devoid of outbound context, the threads are frequently confusing or meaningless.
The workaround I came up with is to create a new Smart Folder that includes Inbound email and Outbound email (Inbox and Sent Items). The Smart Mailbox, in threaded mode, correctly handles putting all of the email together. The little caveat is that outbound email that was not a reply to something appears in this view too (it does not appear in Gmail), but I think I actually prefer that mode of operating.