Hey Jerry, and Razbo,
I've run the same kind of pedal for about 10 years now. I think it really depends on the type of stuff you're playing. For most country I use it on the solos, for the blues and rock I use the sustain. It may not be the same for you though, I know some guys that use it more to boost their leads.

The most important thing is to make sure you have the amp tone you want without the pedals. To me pedals are like spices on food, they're not the main course. I'm all about tone, I want that beefy, full tone that makes you think you need to turn down a little. The thing is to do it without cranking the volume, as you add the tone you'll increase the volume. When you get that sound then add the compressor/sustain pedal, set the tone at about 1:00 to start. Set the level to where when you stomp it, your volume doesn't increase. The other settings will depend on what you're playing.

Have a good one, and let me know how it goes.