I have one and love it, some do the used Ford Taurus fan thing. I bought a new Flex-A-Lite 180 Extreme. Flows 3300 cfm all the time, no matter what the engine rpm is. I was having an issue when crawling slow off road and when stuck in heavy traffic in 95* heat with the stock clutch fan. When you ain't moving fast, the clutch fan ain't spinning fast enough!
So I did it more for that than the fuel savings, but noticed a nice little 1.5-2 mpg increase when I went to it.

The only documented issues I am aware of are on bigger trucks, like Diesels where the e-fans aren't beefy enough to cool them down when towing a LOT of weight. But I've towed a 7700 lb. tractor with no issue whatsoever.