They work great. I use them a lot on snow plow trucks. Snow plows are murder on automatic transmissions due to low airflow across the cooler (plow in the way). I like the ones with a thermostatic switch to control the fan.
I always add an external cooler in addition to the factory cooler in the radiator. I don't recommend replacing the factory cooler with an external cooler. It should be in addition to the factory cooler. The flow should be: out from the transmission into the radiator, then out of the radiator and into the aux. cooler, and then out of the aux. cooler and back to the transmission. (FWIW, the lower line on a 4L60E is the "out") When you're done, just make sure you have good flow through the whole system. the return from the cooler is the lube (and cooling) for your gear train.
1995 Camaro Z-28, M6, Dynamic Customs 396 stroker, Scat forged 3.875" crank, Scat Ultra Q-Lite 6" H-beams, Icon forged pistons (4.030"), splayed 4 bolt w/ARP studs, 12.25:1, AFR 210 Eliminator heads (2.08"/1.6", 58cc chambers), Comp Ultra Pro Magnum XD 1.7:1 rockers, GM847 cam (234/242 .611"/.632"), Edelbrock LT4 air-gap intake, 24x conversion, LS1 PCM, TPIS 58mm throttle body, Bosch 42# "Green Giant" injectors, Kook's stepped LT's, custom stainless 3-1/2" ORY, Borla ProXS muffler...