They're really best for folks who are basically healthy with no chronic conditions and no pre-existing conditions. It's really a safety net for something unexpected, not a plan that covers ongoing things. For example, they'll cover cancer the first time, but some don't if it recurs. Or like Katherine experienced, if you have a chronic condition that requires frequent medical care and ongoing medications, it's going to end up being expensive for you, but of course not as bad as if you had nothing.
We looked at all the plans like this three years ago when DH first considered retiring early, and frankly they probably wouldn't take us because of him, and if they had, a lot would not be covered.