Nps's summary is excellent.
I have a BrakeSmart and absolutely love it. I haven't heard of anyone who owns a BrakeSmart who didn't love it. The only two downsides to a BrakeSmart are the cost and the backorder problem. They just can't keep up with the demand, even though the BrakeSmart is the most expensive controller available (that I've heard of).
It's about $350 for the complete system. As I recall, the controller is about $200 and the required wiring harness is about $150. If you want to use the controller on more than one vehicle, just buy a harness for the other vehicle and move the controller. This also allows you to keep the controller when you sell your truck.
The BrakeSmart taps into the brake hydraulic line to measure actual brake pressure in your truck. Don't worry; it doesn't interfere with the ABS or any other truck brake function. (I have four wheel ABS. ) It sends a trailer brake current that is proportional to the truck's braking. You almost can't tell you are towing a trailer, it's so good.
The BrakeSmart will control 1-4 axles (2-8 brakes) and gain can be adjusted for an empty flatbed or a loaded fifth-wheel. I have played with gain adjustments and haven't found it necessary to do it.
Their customer service and warranty are top-notch. My first controller had problems, and BrakeSmart worked with me until the problems were fixed. They sent a replacement via overnight express and didn't require the original unit back until I one that worked. By the way, the problem I had has been fixed in newer versions.
To be sure you don't have any problems with the controller, I recommend towing your trailer for a short trip before planning any vacation to be sure that your controller works properly. This is probably good advice for any modification, including wiring or hitch.
You will not be disappointed with a BrakeSmart, and you'll recover from the expense long before you sell your truck. I don't work for BrakeSmart.
Loren