Are they good? Yes and no. I think that most types of creative exercises that get your brain working are beneficial in some way... provided you put some legitimate thought behind what you're doing. Slapping some Proxima Nova and flat buttons on an existing site for the sake of getting some Dribbble likes is pointless; taking the time to sketch, write, and generally put some thought towards existing problems and potential solutions is not.
Are they good? Yes and no. I think that most types of creative exercises that get your brain working are beneficial in some way... provided you put some legitimate thought behind what you're doing. Slapping some Proxima Nova and flat buttons on an existing site for the sake of getting some Dribbble likes is pointless; taking the time to sketch, write, and generally put some thought towards existing problems and potential solutions is not.