OS X Mail Client

Recent issues with Mail.app have me thinking I need to branch out and try another mail client to hopefully resolve the issues I am having with threaded emails. In the past I have used Thunderbird, and Sparrow, I never really gave Sparrow too much of a try so that will probably be the first I test out again.  I am curious to what Mac users are using to manage their email so I can give some other mail clients a try.  If you wouldn’t mind sharing what client you use and why that would be great, and if its not on the poll please share a link.