I've used AT&T, T-Mobile, Sprint and Verizon. Hands down, nobody has the coverage Verizon does. They may be slightly more than some other carriers but it seems to be pretty negligible when comparing plan to plan. On the other hand, it doesn't matter how much you save if you don't have service. We're currently getting a 22% discount because of a deal with the wife's employer and a 10% "Loyalty" discount. Even so I still pay $95 per month for 2 smartphones with unlimited talk/text and 3GB of data (my company gives me a $30/month phone allowance so I actually pay about $65).
The coverage in Colorado was excellent. It's even good here in the outback of Idaho. From talking to some people here it's the only carrier that even has service outside most cities.
I've only needed customer service with plans and such or to activate a phone. Verizon CS has always been quick and helpful for me. I've never had an issue with phones or anything. Currently have an S5 as well. My favorite phone so far.
In my experience AT&T would be next best with coverage...can't speak to their pricing.