I also use clear context , there is a free cutdown version , and a pro version .
I tried the pro version for 30 days and then bought it.
It is an excellent application for processing emails, I use it mainly for filing customers emails ito various categories very quickly ,
Like the GTD add in it supports delegating , deleting , tasking etc from emails.
I do not really use the colour coding feature or prioritise emails.
I convert emails directly to tasks and then apply the category from that.
For projects I use another addin : Jello gtd