Yeah, they are pretty worthless. I had 2 of my houses zestimates fall by >20%, go back up >20%, and fall by 20% again all in the course of about 2 weeks.
Remember that Zillow doesn't really know much about the condition of your house or the desirability of the location/view/whatever. So when a 3/2 down 2 block away with boarded up windows and a family of raccoons sells, it looks to Zillow like it's a great comp. Likewise when a really tricked out place sell, Zillow only knows the basic info, so it makes your house look more valuable (even though it probably hasn't changed).
-W