After several months doing tests about SPPM, I think I have found the main source of issues for this algorithm, and I have explained them in the tutorial below, with other ideas and concepts in order to produce good results. I think SPPM is a really interesting algorithm and quality wise is much more capable that path tracing and PM + Final gather. It is a small tutorial I hope I can expand in the coming months. Also I will post here regularly several scenes using this rendering integrator. Regards.
http://www.yafaray.org/node/816