I've never really liked mail...I originally came from a work background that was all Windows based, and heavily Outlook centric...everything was done using it. I don't want or need it, and although I have two copies of Office one per Mac, I have used Thunderbird for a few years now, and I like it.
You can choose the plugins you need, rather than be confronted with an overblown option laden email client...with T/Bird you grab the one you want and your good to go....I can't really see me going back to Mail unless Apple make wholesale changes and improvements. Having said all that, for folks who just want a basic, functional email client then I guess it does an adequate job...my fiancee uses it and has no issues with it whatsoever....It does;t hurt to try, as Thunderbird is a free open source app anyway.