Thunderbird always seemed 'windows-esque' to me. I prefer Mail, it's simplicity and obvious OS integration is key.
I use Mail to manage my wife's account and my own, we have them both running in the same instance of Mail (no switching users to check mail). We then use Dockstar to get individual notifications (I get a red circle on the dock icon when I get mail and my wife gets a pink heart (sappy I know)).
Just to echo what has already been said, if you have an iPhone, you'll appreciate the OS integration more.
BTW, you can begin to type an e-mail address into the 'to' field and will autopopulate with people in your Address Book if that's any good for you? I don't see the need to have Address Book open anyway.