I've been using the EMX512 Powered Mixer for the last year.  I think it's a good piece of equipment for a good price.....BUT, Im my opinion the one knob compression is basically worthless. 

    It does have a small compressive effect that is no where near as effective as a real compressor.  A person with better sound finesse than myself may get it to be more usefull, but I really doubt it 
 
 
That being said, I still think you made a good purchase as long as the one knob compression wasn't a big buying point for you.
I wish that yamaha had included inserts on at least a couple of channels 
 
 
I would set it at mid scale as mckyj57 has suggested.  I'm sure it helps somewhat, just not as much as I would like.  But for such a light weight unit, with all it's features, and, a good price, you can't expect the world 
