We are with AT&T most of our family and friends are on AT&T so we can talk mobile to mobile without using any minutes. The do drop calls from time to time but have very good coverage range all over the US and Canada. I have placed calls in all 50 plus Canada with very few problems and have even done so over seas calls.
I was with AT&T for a long time, then switched to Verizon, and recently made the switch to T-Mobile. That last switch ended up saving us about $50 a month, and with T-Mobile, I actually get reception in my house. With Verizon calls would drop if I was anywhere in the house, all the time.
I've been with Sprint since I was a kid and my parents got their first cell phone. Since we've been with them that long, it's a great deal! I really like it a lot, actually. The only reason I'd ever consider leaving is if I suddenly really wanted an iPhone. But I'm happy with my crackberry for now.
I greatly dislike AT&T, which is who covers my area. I was using another carrier that covered the area around my old apartment, so when I cancelled my contract, I just went with T-Mobile, which is who my unlocked phone came from originally. I had to lie about where I live and go pretty out of my way to get service, though. My brother is always complaining about his AT&T prepaid service, so I don't feel interested in using it. Mine works great.