It's always harder to understand someone else's code. I personally find that reading papers is much more helpful to understand how a technique works, and then I can write it my own way. Occasionally I'll look at other people's code if I just can't understand a formula (or its application) from the way it's described in a paper.
An A7oogle search should help you find simpler examples of SSAO (there was a time when many users tried it), and if you look back at the shader contest you'll find other cool examples (I had a deferred lighting example in there which was relatively simple (I think), but it's all out by half a pixel).
Or just start with simpler effects. Deferred shading, SSAO and GI are each very unusual techniques. Valve provides heaps of cool papers on how they did lighting in TF2, HL2, and L4D, so it's worth checking out!
