Thunderbird

I've mostly moved to open source software -- Firefox, Vienna, Adium, NeoOffice, etc. The big exception is iTunes where Apple has me neatly locked into their software since I'm locked into their iPod. The other exception is that I use Apple's proprietary Mail program since it's always seemed fine. Is there any good reason to switch away from it and start using Thunderbird instead? Or, conversely, is there any good reason to avoid Thunderbird?

Oftentimes, I'm not exactly sure what it is Apple has invested in the proprietary software they bundle with their computers. Why bother with not charging for Safari when they could just not charge for Firefox? iTunes, I understand, is part-and-parcel of the iTunes Store revenue stream . . . but most of the rest seems pointless.