I use nginx to proxy cache the Arch Linux package repository transparently. It's fairly easy to set up, and enables nice features like contacting a secondary mirror if the first one is down, or when multiple requests hit the same resource, all are blocked waiting for a single merged package download, so the proxy will not make the download multiple times if I run pacman -Syu on my 18 machines in parallel. And it's all just 20-30 lines of nginx config.

It's not transparent though.