Haven't used the C6 but I have the C4 in my bundle and it is quite a useful plugin. I use it more for corrective purposes than effect type compression though.
Once the light bulb goes on you'll understand how it works, but basically it just lets you compress certain frequency bands independent of each other.
For example, I used it yesterday while mixing a jazz record. There is a song with a muted trumpet, and those can get QUITE shrill in some areas of the top end. Even though I recorded it with a Royer, the warmest mic we have, about 1/3 of the notes were still pretty annoying. Added a c4. bypassed all but the third section. Set it to compress between 4.5kHz and 8kHz, pretty fast attack, then adjusted the threshold and gain until it sounded balanced. So half the time it's not doing much, then when he gets to the annoying higher notes it pull only the high frequencies down that I've set. Bit like a de-esser in this case, but it has many more uses.
Andy's setting is basically doing the same thing but in a different frequency range on guitar. You'd use this if you don't want to eq the entire sound, only when it does something you don't want it to do. Like that trumpet, or palm mutes on a guitar. Eqing out 3 db at 200hz might take a bit away from your tone, but might be perfect when it builds up in the palm mutes. enter the multiband comp.
