Personally, I switched from Verizon in early 2007. I thought the whole service was crippling-- I traveled out of the country frequently, and needed to rent a phone. I hated that I couldn't use bluetooth the way it was intended. File transfer was a pain in the butt. And it was the most expensive of all!
I now have a Blackberry on T-Mobile and I love that I have the freedom to install programs and alter things.