Obviously the benefit of running a PHP based source is that it is very easy to update and add packages. For me, I run what's called a sandbox or test source, where I fully test each and every package (install, uninstall, upgrade) before I add it to my normal source.
Edit: You need to have boatloads of bandwidth on your server. To put it in numerical terms, my source which is a very small source pulls about 8GB to 14GB per month in just the 40k plist file (my files are hosted by google). STE says he pushes about 750MB to 1GB per day in repo list updates, that is not counting the actual transfer for packages which I'd guess it a huge number.