Despite there being a lot of answers already, they are somewhat repetitive and I would like to take a different tack. Unit tests are valuable, if and only if, they increase business value. Testing for testing's sake (trivial or tautological tests), or to hit some arbitrary metric (like code coverage), is cargo-cult programming.
Tests are costly, not only in the time it takes to write them, but also maintenance. They have to be kept in sync with the code they test or they're worthless. Not to mention the time cost of running them on every change. That's not a deal-breaker (or an excuse for not doing the truly necessary ones), but needs to be factored in to cost-benefit analysis.
So the question to ask when deciding whether or not (or of what kinds) to test a function/method, ask yourself 'what end-user value am I creating/safeguarding with this test?'. If you can't answer that question, off the top of your head, then that test is likely not worth the cost of writing/maintaining. (or you don't understand the problem domain, which is a waaaay bigger problem than a lack of tests).
http://rbcs-us.com/documents/Why-Most-Unit-Testing-is-Waste.pdf