guitarjesus, that old statement is true enough in its clever way, but also misleading and totally useless to poor OstiaMan. First off, usually there is a discernable reason that it sounds good -- that is, people noted that it sounds good, asked themselves "why," and knew enough about music to see why and extend the idea, finding more stuff that sounds good for the same basic reason, and allowing them to explain it to others. Learning this allows you to suss it out and get there sooner, without having to either spend a million years trying to play every possible combination of notes often enough to eventually recognize all the ones that sound good and play just those and leave off the "bad" ones.
The other point I have to make is that some of these more "out" sounds don't sound good until you train your ears to hear them in places that they work. So if you only go by what "sounds good," you might never kearn to appreciate the more piquant delights of the diminished and augmented scales and their close neighbors like the lydian dominant scale...
And Toad Rush, he already knows what you are saying, and he said so in the post -- he wants to know what other uses the diminished scale might have.
OK, OstiaMan, here's some help.
The diminished scale can be used over dominant chords. The diminished scale built a half step above a dominant chord's root works (for example, Abdim7 is the same notes as G7b9 without the root). Moreover, the symmetry of the dimished scale (the same notes are contained in Ab dim, B dim, D dim, and F dim) means you can move diminished fingering patterns around by minor thirds up or down the neck and the resulting notes are all good. This implies there are only three different diminished scales.
The augmented scale -- the whole tone scale, a 6-note scale as opposed to the more familiar seven-note diatonic scales -- also works as a dominant substitution. It contains a sharp 5th, natural ninth, and the dominant 7. There are only two whole tone scales -- the remaining notes not in one are in the other and vice versa.
Be aware, just playing these notes over a dominant chord does not guarantee you'll be playing anything that sounds very good at all, espcially in isolation -- which it the second comment I made above in response to the "if it sounds good, it is good" statement. You really have to work to "hear" these sounds and to use them in a musical way -- you have to slip into them and back out of them appropriately as they harmony directs. They are almost always used as a way of moving back to the tonic (or to the tonic of a new key) and sound strange or wrong if they don't resolve just so at the right time.
