My understanding for using the Prodigy controller on the X5 is due to the method required of picking up the brake light signal. As the brake lights are triggered by a hall effect sensor on the pedal (as opposed to the usual method of a mechanical switch) you can't just tap into the sensor like you do on most vehicles. You are picking up the signal on from that 'pin 12' over in the passenger footwell area. Not sure what the exact reason is, but it has been determined from trial and error by other people that the Prodigy is the only controller that works in the X5 without screwing something up.
I haven't installed one yet, but it is on my list, just haven't gotten around to it yet.