In her latest post Anna Filina proposes that full test coverage is an impractical way to measure the quality of your software. It can provide a false sense of security, even if the tests are poorly written.
Many developers claim that to achieve high quality software, developers must create automated tests that ensure that all possible execution routes have been covered. This is also known as full path coverage. I will argue that different types of software require different testing approaches and that full path coverage is impractical in almost every case. Too many tests simply create clutter.
She looks at how it's impractical to expect that all tests will be written efficiently or even correctly. Even simple tests are enough to show up on code coverage reports but may only be painting part of the picture. She also notes that not all software can be tested the same way - things like APIs require different testing skills/methods than something like consumer software.
In the end, there are no exact rules on how much to test. The most important thing to keep in mind is that writing tests for the sake writing tests is futile and costly. [...] Focus on building great software. Tests are a tool to make it better. Just don't overdo it.