Until recently, I've been using Apple Mail for all of my email needs. Although we typically loathe Apple Mail for it's overall bugginess in my workplace, I've been apathetic and used it for nearly 2 years. In doing so has warranted some abuse from my coworkers, all of whom swear by Thunderbird. My hesitance to switch was purely based on my willful ignorance toward other applications.
Derived from Thunderbird and Mozilla labs, Postbox is a cross platform e-mail client that brings some of the great features from Gmail to the desktop. Features like tagging, topics, conversation view, tabs, and smart search made this an ideal replacement for Apple Mail. Setting up new accounts was also easy; if you're familiar with Thunderbird, you should feel right at home. Within an hour of installation, I had set up 4 IMAP accounts (2 Work, 2 Personal) and set up mail filters and synchronized additional IMAP folders.
I'm still running beta 10 however the newest version is said to have significantly decreased CPU and memory usage. AOL Mail account support (ugh) has been added as well. The full list of bug fixes and improvements can be found here.