I've been with Verizon since they were Bell Atlantic Mobile (almost 11 years now) and I won't change carriers. My husband had Sprint a while back, it would literally take him
days to get voice and text messages. Once, I tried calling him all day to get him to come home from work early (I had a stomach flu and my kids at the time were 4 and about 8 months old) and he didn't get any of the messages until two or three days after the fact. He switched over to Verizon not long after.
My BIL has AT&T and his calls get dropped about half the time.
Verizon? I went skiing at End-of-the-World, Vermont and on the ski lift, at the top of a mountain, I had perfect service. Awesome.
I'll be getting a new phone in the next few months, and much as I would love a Droid, I can't justify the cost, so I have to do some serious comparison shopping.