ENTERPRISING = QUICK WITTED ≠ OPPORTUNIST

If you want to praise people for taking legitimate and skilled advantage of opportunities that spring up, it is better to call them “enterprising” or “quick-witted.”
          When applied to people, the label “opportunist” usually has negative connotations. It implies that the people so labeled take unprincipled, unfair advantage of opportunities for selfish ends. Opportunistic people are often also regarded as exploitative. The term is often used to label unscrupulous politicians who seek to manipulate voters in their favor by exploiting certain issues or opportunities in an unethical way. 
         Sports commentators who call the skillful interceptor of a pass in football an “opportunist” are misusing the word. 
        The specialized meaning of “opportunistic” in biology does not cause problems because the people who use the word in this sense know what it describes: the ability of a species to exploit a previously unexploited ecological niche.
Source: http://public.wsu.edu/~brians/errors/opportunist.html