If configured correctly, varnish's cache is lightning fast, and incredibly flexible. Nginx + memcached can make things really fast, but I've seen cases where th

If configured correctly, varnish's cache is lightning fast, and incredibly flexible. Nginx + memcached can make things really fast, but I've seen cases where the time to first byte can be 20-40% faster with a well-tuned Varnish instance (we're talking milliseconds, of course). (Sometimes, I imagine this may be due to the fact that bleeding-edge performance people are more familiar with Varnish, though...). Nginx has been getting better, and I'm excited to see what happens over the next couple of years.