I used to use KMail for years. It had some issues especially with IMAP accounts but worked fine for the most of the time and problems could be easily avoided by careful usage.

The Mail however in Mac OS X is completely different story. It is quite stable and better at using the IMAP accounts but it is consistently annoying in certain places.

First of all attachments - when I forward an e-mail with an attachments they are missing after sending the message even though they are showing fine during message editing. It happens always when forwarding a message. Therefore when I have to forward a message with an attachment I just:

1. save the attachment(s)
2. create a new message
3. add attachments
4. send the message

and... attachments are missing too but not always.

Another annoyance - I want to move a message to a different folder: I click on the message and then try to drag it to the folder. Instead of moving the message it selects all the messages in current folder.

I really hate the program. Switching to something else....