I have one. I have not used it in a long time. It throws a check engine light, but perhaps the biggest issue I have is that if you want to actually utilize the full capability, you must shift both the rail, and the floor shifter to get engine braking.
If you leave the trans in "D", and down shift the rail, you'll only get engine braking in 3, any lower and the valve body is not mechanically lined up to supply compression braking. So you have to shift the rail down, and the floor shifter.
If you want to just use if for gear selection, it works fantastic. Put the gear in "1-2", and you'll have full control of those gears with the rail. But then if you want to go up to 3, you have to shift both.
Final disclaimer- If you use the shifter, the tcu goes into limp mode. When that cel goes on, I have tried to "turn off" the rail by using my switch, and the tcu will no longer respond. It'll accelerate from a stop in like, 4th gear. If you shut the engine off, and cycle the key back on, the tcu powers back on, allowing normal operation, with a cel.
It works great for choosing gears, but I'd love to see the winters kit.