Very rarely, gasax - they don't usually have TXPDR on.
Many more military aircraft seem to have txpdrs turned on. My PCAS / 496 has picked up Lynx & other helicopters, tutors, various fast jets, jetstreams, out of the mil base we're based near....as well as the coastuard helo, the air ambulance, police helo, and CAT (obviously).
I believe that although maybe only 50% of GA have txpdrs, those who are either based at, based near to "big" airfields and CAS normally do, and that those who tend to do cross country trips probably will have and use a transponder.
I try and go high - go at 7000 + if you can,lookout and use any available technology to help.