The question about magic effectiveness depends completely on the belief of the one involved with it. One cannot expect a miracle if you don't believe in saints.(Alejo Carpentier in the introduction to his book El reino de este mundo)
David, I'm assuming you mean sympathetic magic as defined by Frazer in The Golden Bough? While I don't agree with Frazer, partially because I find his approach too "Anthropological" - in the old usage. He lays claim to objectivity but brackets out his own social and intellectual context (personal beliefs, customs, memories) and the way his own culture has shaped his 'ways of knowing', while proceeding to observe people, their culture and practices, like objects.
For example, Fraser's discussion of the "primitive magician" states, 'to him magic is always an art, never a science; the very idea of science is lacking in his undeveloped mind. It is for the philosophic student to trace the train of thought which underlies the magician’s practice; to draw out the few simple threads of which the tangled skein is composed; to disengage the abstract principles from their concrete applications; in short, to discern the spurious science behind the bastard art.'
The tribal shaman, healer, lore holder, works consciously and uses spiritual science - science based on the laws of spiritual reality rather than laws of physical reality - to affect healing, the environment, and so on. Art itself has scientific aspects - laws of musical composition, the relationship between primary, secondary, tertiary, etc. colours. In my view, Frazer's separation of art and science does not hold.
I am convinced that sympathetic magic is an intrinsic part of being human, that we relate to it and apply it both unconsciously and consciously in our everyday existence - albeit not at the level of, for example, the tribal shaman. We wear wedding rings - the circle as a symbol of eternity - to 'cement' an enduring relationship; we wear team colours to football etc. matches to 'support' our teams victory, we treasure mementos left to us by deceased people we loved because these things - once in their possession - allow us to feel close to them, etc.
To finish my somewhat rambling answer: yes, I think sympathetic magic still works in this modern era - we are still human.