Very true Rick.
The MetroMix is basically the same as HP and Pro Mix etc. I mix the MetroMix, charcoal, and perlite at around 1/1/1. If you can get sponge rock or large perlite, I find it more ideal. I've grown everything but Dendrobiums and Cattleyas in it with as good of results as any other mix, and always had great root growth from every slipper I've tried.
When watering, all you must do is let it completely dry before watering again...and fortunately it tends to dry very evenly. Even when dry, it remains quite humid, so the roots never get that stressed. Species you would normally grow drier like caudatums, praestans, and druryi, I just let go dry another day or two more than I would anything else and use styrofoam peanuts in the bottom 1/3-1/4 of the pot. If in doubt, don't water. Shrinkage has typically been minimal, as long as you use plenty of charcoal and perlite. I also highly recommend Air Cone pots for the mix, as it helps aid the even drying. Ideally, I repot this mix yearly...but in practice it tends to be every other year and so far roots have held up much better than if left in bark for that amount of time.
Jon