LED's work fine but they're not very powerful. I think the newer LED grow lights produce around 40 lumens/watt. Even the latest LED's can only muster around 75 lumens/watt. Anything beyond that is experimental and not yet mass produced.
Until these LED grow lights become cheap to around $20 each it won't be cost effective versus HID grow lights.
HPS works because it is very efficient, your average horticultural bulb does 150 lumens/watt. It makes up for it's lack of spectrum with shear power. Metal halides work but they're less efficient, around 100 lumens/watt. They give off more blue but less red, making them less favorable compared to HPS. Enhanced HPS bulbs meant for growing have a stronger red spectrum, currently it's the most efficient way we know of to produce artificial sunlight for plant growth.
Because of their fundamental design and need for a ballast, HPS technology probably won't advance past where it is at now. LEDs on the other hand are getting more and more efficient. I wouldn't be suprised ten years from now that "UFOs" are commonplace among growers.
Pictures description:
1) Plant photosynthesis chart
2) Light wavelengths emitted by standard HPS, your average security light/street light
3) Light wavelengths emitted by a horticultural HPS bulb
4) A horticultural metal halide conversion bulb
5) Light wavelengths emitted by the common "warm white" CFL
6) Light wavelengths emitted by the common "daylight" CFL