Personal BB-personal e-mail and phone calls
Best feature: seamless and instant e-mail (my old T-mob 6320 wasn't instant)
Second best: Google maps- I love being able to put a search for business and say "Pizza" and be able to find my lunch in any neighborhood in nearly any city in the world (with directions and the phone number, too, so I can order my pie before I even leave the building)... or type in the name of my hotel when I'm hopelessly lost... or be able to never ask for directions again as long as I have an address to enter. Very cool feature.
Third best: The video player- I've uploaded my yoga DVD so I can do it in any hotel in the world, even if there's no DVD player. I can have in-flight entertainment even if I'm flying economy airlines. I can have a laugh while standing in line at the DMV watching a best of SNL episode.
Fourth best: Internet access and everything that comes with it. Whether it's Stinsondog's mobile links to just about everything I could possibly need, or keeping entertained waiting for my oil change with some Fark surfing, or using the Wunderground radar image to let me know what weather is heading my way; it's a great feature.
Fifth best: Using the media player for music- While lacking the capacity and the random feature of my i-Pod, when you don't want to carry two devices, it'll pinch hit just fine.
...oh, and it's one of them-thar' cellular telephones, too.