IMO the only way to get a good read on this is to talk to people you know locally. Coverage maps and such don't tell the whole story, and what's important is that your devices have good coverage where you actually use them.
Other than that, there are going to be horror stories and "X has been fine for 15+ years" stories with every carrier. I, for instance, have been with AT&T for nearly 20 years. I say nearly because I'm honestly not sure when we first got our cell phones, but it had to be sometime between 1994 and 1997. I've shopped around often over the years and just never had a good enough reason to switch. I've never had an issue with the customer service for ATT Wireless, either. U-Verse, on the other hand, has been nothing but trouble for the few people I know who tried it, but they're related only in name and are actually two separate entities.
tl;dr Talk to people in your area. If you can, test out speeds and connection quality from different providers at places you frequent to get a picture of what you would be working with on a daily basis.
