Metal Halide or mostly called MH has been around in the aquarium industry and hobby for many years.  If your looking for maximum coverage for lighting your reef tank Metal Halide lighting might be the best option for you.  Halide lighting offers large reflectors allowing you to cover a huge area on a fish tank or reef tank.  

Fragtastic Reef has personally used MH lighting for over 15 years on specific tanks, especially SPS dominated aquariums.  Metal halide lighting has started to make a come back the last couple years, mostly on systems with large areas that need coverage.  With MH lighting you can grow anything for live corals, however it comes at a cost.  Halide lighting is not energy or cost effective, and bulb replacements should be done every 10 months, they also produce a substantial amount of heat.