This is actually old news—but I thought it was important to be reminded of it. Last year Yahoo's UI blog posted some findings on user browser cache usage. Here's what they found:
40-60% of Yahoo!’s users have an empty cache experience and ~20% of all page views are done with an empty cache. To my knowledge, there’s no other research that shows this kind of information. And I don’t know about you, but these results came to us as a big surprise. It says that even if your assets are optimized for maximum caching, there are a significant number of users that will always have an empty cache. This goes back to the earlier point that reducing the number of HTTP requests has the biggest impact on reducing response time. The percentage of users with an empty cache for different web pages may vary, especially for pages with a high number of active (daily) users. However, we found in our study that regardless of usage patterns, the percentage of page views with an empty cache is always ~20%.
I definitely recommend reading the whole post as there's lots of good information on optimizing your site for performance. This article is just one posting in a series of posts on how to improve your site's HTTP performance.
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