I switched to eM Client after trying different clients and I absolutely love it!
The only two things bugging me are the lack of treads/conversations (which will be fixed in version 5)
and
the fact that the mail I send as a response to a mailing list only shows up in the Sent folder, not in Inbox, which makes the thread even harder to follow.
When you send mail to any group or mailing list you subscribe to, Gmail automatically skips your inbox and archives the message to save you time and prevent clutter. The message will appear in your inbox if someone responds to it or if there is an error delivering the message. If you’d like to view your message, you can find it in Sent Mail or All Mail.