The stock cooler is prone to plugging.
An aftermarket cooler of larger or better design is a good idea if you tow a great deal or really heavy loads.
I personally prefer the self regulating style especially if you get cold winters.

 

YES YES YES YES YES YES YES YES!!!

EVERYONE needs an auxillary transmission cooler on their vechicle, no matter what type of driving you do. In warmer year long climates you can completely bypass the in radiator stock cooler with an aftermarket one. In colder year long climates you should run them in series, but make sure you flush the cooler and lines, trash and debris will built up in the coolers over time.

Also, remove the check valve in the stock cooler line. Over time it can become plugged up and stick closed causing the cooler line to become plugged... Then the transmission will receive NO cooling lubrication. Hard parts will then melt together and you wont have any warnings until its too late.

 

the tubular coolers do not cool as efficiently as the plate style.
plus they tend to be powder coated black so you won't see it through your grill.