This is interesting... At my school we have only 1 quarter of circuits, and we have a quarter of signals/system that is completely separate.
I'm guessing it might be better to teach it in the context of something like circuits, in order to give the students something tangible to latch on to. Oppenheim lays it out almost purely as an abstract subject.... (which--being something of a math oriented fellow--I actually enjoy in a twisted sort of way :)
yeah, I had one 4 credit circuits 1 course and 1 3 credit circuits 2 course. These courses collectively covered all of the stuff on dc resistive networks, transient responses, capacitance, inductance, laplace transforms and s-domain analysis, system theory, fourier analysis and frequency response analysis, 2 port networks and many other things. But I never had a standalone systems class.