I've written CSS for years and only recently started implementing some animations in my designs. I was worried at first when browsers started implementing it, fearing useless and insipid animations would sprawl around the web (like some jQuery ones). But like anything, if used sparingly, animations can improve an interface. Apart from adding life and being "cool" (probably the reason why designers implement them in the first place), animations add interactivity and affordance. Plus, they're less prone to bugs because the code is easier to read and write than jQuery alternatives, and the browsers' rendering engine usually prevents unwanted fluttering and infinite loops from happening.