Les, I moved this topic from the Color to the B&W forum.
PMK as the name implies uses metol in conjuntion with pyro to develop the film and take advantage of the supperadditivity properties of these developers. Other pyro formulations, like ABC use only pyrogallol as the main developing ingredient. As such their qualities and problems are different, IMO ABC pyro yields a greater tonal scale than PMK and the stain is somewhat different than that produced by PMK. OTOH PMK has the good quality of being capable of film expansion by time instead of having to add extra activating solution (Carbonate) to produce greater contrast as you have to do with ABC.
ABC oxidizes much faster both in the bottle and when developing, thus when used in developing strenght it can cause uneven staining (this is peculiar to most of the formulas which use pyro only as the main developer). With PMK this is not a problem as the solution does not oxidize as easily.
PMK lasts in the bottle a good six months, most pyro only formulas do not, I would say 3 months would be the max. PMK is much cheaper to use, you mix the stock solution and use it in1:100 ratio, while ABC and others are used in a 1:7 to 1:20 ratio.
As stated by Donald, PMK is a great developer for enlarging, and ABC and others are mostly better for contact printing. Your fellow countryman Barry Thorton has a very interesting formula based on pyrocatechol, another staining developer, but he has chosen to make his formula proprietary and frankly is way too expensive. If you have more interest in these formulas, look up the formulas Ed Buffaloe has compiled in his site unblinkingeye (there is a link for it on our home page) there you will find formulas based on both pyrogallol and pyrocatechol as well as many variations of the most commonly used formulations.