Cheap ones are time based (ie the longer you push on the brakes the harder it applies the trailer brakes). Better ones have accelerometers to determine how much to apply the brakes.
More info, just in case you're unfamiliar with the subject.
I have this Curt controller in my Suburban.
It is classified as a "cheap" controller since it is a simple time-based unit. I can select the amount of maximum brake force applied (represented by the digital display, in increments of 0.5) via the thumbwheel on the top, and I can also adjust the rate of application via the slider on the side. Other than that, it is a 'dumb' controller. Don't get me wrong--it works great. However, this means I will make adjustments to the total brake force depending on where/how I am towing. On the open interstate, I usually dial it up to either 3.5 or 4.0. If I am at slow speeds such as in town, I will typically drop it down a notch or two (usually to 3.0). During very slow speed driving such as in a parking lot, I may take it down to 2.0. When the trailer is empty, I'll leave it at 2.0 or even 1.5 depending on my speed. Some might say this constant fiddling is a hassle; I just chalk it up to part of the towing experience. (For the record, I never fiddle with the adjustment for the rate of application.)
Having said all that, one of the fancy inertia controllers (such as the one linked above) sure do sound sweet.
