"I almost never test at the view level, and barely at the controller level"

You sound like every Rails dev I know.

It depends on the controller action in question. If it's a boilerplate Rails controller action, then the ROI might not be that high, but if the controller is doing something more specific (assuming it's not doing something that ought to be pushed to the model) then unit testing the action is important, sometimes very important.

Code coverage is pretty much a meaningless metric because it can't say whether the tests are actually good and useful. But it can help point experienced devs into areas of risk that may or may not warrant more attention.