I use two in some cases. One permanently on on my amp, and a boost compressor on my pedal board. I have the one on the amp on just because it's there and it has a pretty light. I use the boost to even out the sound whilst using an octaver, but it acts more like an eq than anything else.
Other than that I can't really hear any difference when using one. I'm sure it's there, but to my ear there's very little, if any difference. I must admit, I don't really get the compressor thing.