In terms of compression i'm in the RayC boat; the older i get, the less i use. Vocals and snares usually get some, maybe kick. I steer clear of it on electric guitars and acoustic guitars and would rather automate the volume if needed.
Saying that, i remember vividly using a comp on a crazy synth part with an arpegiator on it a while back to balance the the root notes with the extra notes. it sticks out in my memory because, as someone else mentioned earlier, alot of synth/key parts i come across are already fairly compressed so i normally steer clear of adding any more.