Personally, I consider them vastly overhyped and of marginal value. The idea sounds great, but when you get down to it, there's really only a handful that common and useful enough to be worth remembering (and possible to remember).
I'd say their net effect is negative, due to the hideous overengineering perpetrated by people newly enamoured with the concept and trying to cram as many patterns into their code as possible. Perhaps even worse is the Maslow's hammer effect leading to bad design because instead of finding the optimal one, the developer remembered a design pattern that fits (badly).
