What is it you want from an email client that Mail is not providing? When I first switched to Mac, one of the first things I installed was Mozilla's Thunderbird email client. But Mail has improved a lot, and is every bit as good now.
There is that one little pesky bug in Mail for Mountain Lion... where if you use rules to move mail from the inbox to a folder (or move the mail manually), when you reply, the replies come from the default address within Mail, and not necessarily the email address to which the original message was sent.
I know Comcast has only ever offered pop3. Of course, I've always recommended people not use their carrier email so that when they get annoyed with the speed of their ISP and want to change, or move and can't get their old ISP, they have a reliable email.
Generally I like Mail, but as some are contending here, it isn't perfect. I was just wondering if there's such an email client that rises above all, kinda like a SupermanMail or a WonderwomanMail, or better yet: a Bugs-bunnyMail...