Gary,
There is basic information in the help from memory, but I think you need to spend some time understanding what it is doing. I think Craig Stark images at short focal lengths and has vey tight stars in his guide scope / guide CCD combo - judging from this image of his results he posted
http://files.getdropbox.com/u/521680/PHD1108atest.jpg
I know that using a single guide star and judging by size and frequency of corrections - PHD is at least as good as MaximDL 4.56. I am unsure how it compares to MaximDL whenyou have multiple guide stars selected (this may even out seeing errors).
Craig has such low RMS 0.2 compared to mine 0.8 I ponder is this sky quality (seeing) and/or focal length. Unless we highlight the challenges of longer focal length imaging I ponder if he mainly tunes and defaults PHD settings to optimise short focal lenght performance.
PHD can work very well - you just need to experiment some to get its parameters right. I do think sitting with Excel and charting the performance - plus by the human eye ranking the tightness of your stars is the best, most systematic approach to tuning PHD.