All of these responses are so interesting. I guess I'll throw my 2 cents in ...
I am on Verizon, and I'll tell you why.
When I got my first cell phone, I got it on the only service that was available where I live. I think it was called US West Cellular. I was happy enough with the service, but I didn't use it a lot (super expensive back then). Then, Airtouch bought US West. I didn't experience any big change in service, but I still didn't use it too much. I think it was after Airtouch that it became Verizon. By that time, there were other options becoming available, but I had no reason to change.
Our Verizon rep had become a good friend (not only did we have a personal account, but each of us was responsible for our business accounts). When he told us that he was going to be heading up the new Sprint push into our area, my husband and I were wary (my husband had had a bad experience with Nextel). However, because he was a friend, we moved all of our phones (personal and business) to Sprint.
Everyone here has a different priority. When you live in a remote area, coverage is the key. And, Sprint just didn't have it. They kept promising better service, but I couldn't even make/take a call while I was sitting at my desk at work. We hung in there for several years. I even had my first smartphone with Sprint (Kyocera 6035). However, once our friend left town and number portability was available, we couldn't get out of there fast enough.
I agree that Sprint has the best pricing on their plans, but it doesn't do you any good when you can't use the minutes because you don't have service. What I missed most when I left Sprint was my Samsung I500. I LOVED that phone, and it was a major change (in size) to go to the TREO (what a brick ... but it worked like a champ!)
Everyone has their own reasons for why they are with the carrier they have chosen, whether it is for the plan, the service, or the device. This has been mine. I don't plan to leave Verizon any time soon. So, I just have to pick the (Verizon) device that is right for me. Right now it is the Pre+, but who knows what it will be a few months from now.