Allan,
I am using "Edit in Textmate" for most of my emails now, but I am finding out that there are some caveats that will kill your email message; namely switching between windows and/or applications too often results in your text being lost and not placed back into the email when you close the TxtMte-Window. Right now I am placing the email message into the clipboard as a backup, but clearly there is something odd here ;)
Dan