We just spent a month in the Rockies. We have US Cellular and for the most part, we had no service. If we had any service, it would drop about half of our calls. My wife's phone worked better than mine, but not much.
In southwest Wisconsin where we live, US Cellular has the best service. I guess months a year with good service is good for us. I would personally leave my phone in the truck while we are on vacation. We checked in with family about twice a week by email.