Well said, Winslow! Let me also point out that in your worst case scenario, which I agree with, when you slow down (after pulling a load) with the stock fan clutch, the fan turns at idle rpm - little air flow thru the radiator - not good! On the other hand, the electric fans are thermostatically controlled, and if your coolant temp is 185 (or whatever you set) the fans deliver the full 4500CFM - at idle. If you're warm you might want to rev your engine a little to augment water flow thru the radiator, but the fans are delivering fully, independently.
I wrestled with the purchase of this fan for sometime because no vendor was willing to say this fan, or any other electric fan, could cool our cummins'. Well, two plus years later I can report that this fan cools way better than stock, and has not let me down. When I purchased this fan I even bought a back up fan support off Ebay, and always kept it aboard, with the original fan/clutch assembly. No more.
The install is a little snug, but IMO well worth the effort. For what it's worth, I gained confidence to try this after reading on our good old TDR about other members running without any fan whatsoever.