If you're in the market for a new email client to try, I mentioned Postbox a few weeks ago. Late yesterday, the Postbox team released beta 8, which features an improved conversation view, and new and improved support for QuickLook, Flickr, Hotmail, and Growl.
The beta also fixes many bugs found by users (including myself) and performance enhancements. Also, Postbox is heeding user suggestions to become more Mac-like, including keyboard shortcuts in menus, improved notification for unread messages in the dock, and the aforementioned QuickLook and Growl integration. Full release notes are available on the Postbox website.
Performance appears to be about the same -- which is to say pretty good -- and indexing on my 2006-vintage black MacBook still occasionally ramps the fans up to hurricane strength. I've been using Postbox as my primary email client for about three weeks, and its held up to my demanding workflow, but still has a ways to go before it becomes the killer email app.
Still, if you're a high-volume email consumer, Postbox is a solid option for you. Postbox is free (and will continue to be free, so far as we know), universal binary, and requires Mac OS X 10.4 or higher.