I, too, have heard mixed reviews from those converting to electric fans. A couple of things to consider:
1) A properly functioning fan clutch will allow the fan to freewheel when it's not needed, and only engage the fan when the demand is there, just like an electric fan. I often wonder how many of those who claim gains from an electric fan had a bad fan clutch and the fan wasn't freewheeling.
2) As a thermodynamicist wannabe: Energy can't be created or destroyed (1st law of thermodynamics). An operating fan is going to draw power away from the engine, one way or another. 2nd law of thermodynamics tells us that real processes are less than 100% effecient at converting energy. So the question is: which is more effecient at transferring power to the fan: the mechanical fan clutch or the alternator. I haven't seen any controlled experiments to answer this question, but have to wonder if the engineers at GM have.
3) It seems that the biggest mistake to be made in the conversion to an electric fan is to cheap out and get too small of a fan. Many "failed" conversions were a result of the engine overheating from an inadequate fan. I recall one conversation where the "failed" conversion was from a guy living in Phoenix, and the "successful" conversion was from a guy living in Minneapolis. With that in mind, a tow vehicle is not a place to be installing 2nd rate cooling equipment.
This seems to be a long standing debate that will probably continue as long as there are internal combustion engines. I personally don't feel there is enough solid, positive evidence to make it worth the cost and effort to make the conversion, but I'm sure there are those out there who feel differently.