A commercial client wants to convert his incandescent exterior lighting (mounted on building) to HID. In theory, the energy savings will pay back the cost of the changeover in a few years. I have heard from several contractors that the life of HID lamps is often much shorter than advertized. Due to the cost of relamping, this would significantly extend the payback period. I would appreciate the comments of others with first hand expereince in this area.

jsherrard,
Welcome to ECN!.
HID lighting in general would make a better option than an incandescent source, any day.
A standard Metal Halide lamp will give 15, 000 Hrs of light and a SON lamp will give up to 20,000 Hrs.
Also, a HID lamp is generally more robust when it comes to vibration and voltage surges.