The performance of HTTP cache servers varies dramatically from server to server. Much of the variation is independent of cache size and network topology and thus appears to be related to differences in the user communities. Analysis of a range of user traces shows that, just like caches, individual users have highly variable hit rates, Zipf locality curves and show strong signs of long range dependency. In order to predict cache performance we propose a simple model which treats a cache as an aggregation of single users, and each user as a small cache.