I hate having things "to do" in both my email and RTM. As a result I forward emails for action to RTM (using the email add feature and Smart Add in the email subject).

However... two down sides...
Firstly, email attachments are not included.
Secondly, the action may often involve replying to or forwarding the email and there is no easy link back to the email.

It would be great if the RTM task included the URL of the Gmail so I could simply click on it to open the email (back in Gmail). Which also solves the attachment problem too.