I had my teeth whitened a few years ago and they are still pretty white. I don't drink coffee, tea, or any other beverages that can stain the teeth and if I do have something I always have a piece of the gum with Xylitol in it since it is supposed to clean your teeth right afterwards.
The procedure made my teeth extremely sensitive afterwards and I didn't really eat for a couple of days.
Overall, it is worth it but also important to watch what you eat in terms of color and have gum or brush right after you eat something that may stain your teeth if you want the effects to last. Yeah, kinda a pain