LOL I do actually make a diff of the source code for each new version I install, and give it a quick review to check whether there's nothing suspect and it corresponds with the changelog. 

I haven't written my own bitcoin client yet, but I've made my own implementation of several parts of the Bitcoin protocol (e.g. a parser of raw transaction data and a base58 encoder). I'm starting to become familiar with where I can find what in the Bitcoin source code.