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Great interface, simple operation, easy to customize.
I use Growl on my Mac and thought that I would like GrowlMail, but I didn't at all. It just opened a bunch of windows and cluttered up my desktop. So I tried Herald, and it's great. It just opens one window, but it's easy to flip though multiple messages. The toolbar is really well designed, allowing you to mark a message as read or go to the full msg, or directly to a reply in Mail.
Pros: Simple, customizable, smooth
Cons: None yet.