I built a pretty simple basic harness that was a bit more fancy than that interface.khm.de link. See here
http://www.diystompboxes.com/smfforum/index.php?topic=96200.msg835378It sounded ok on my breakboard, though there was some whine. It's super easy to make delays, tremolos, bit crushers, and sample rate reducers. Beyond that you start to run into the limits of what you can calc between samples. Then it becomes a balancing act between algorithmic complexity and sample rate/depth. Ie., if you sample at 20kHz instead of 44kHz and take 8 bits instead of the full 10(?) the ADC will give you, that frees up cpu cycles (and memory) to do fancier things.