Good luck with that. I tried to gather some information on it a couple of years ago to no avail. all that is out there is propaganda.
I think you'd be better off subsidizing mother nature as opposed to replacing her. In that circumstance, MH lights would be a better option. With LED's, as I understand such things, you have to place them very close to the plants. That makes them a very tedious task if you're just going to supplement the sun.

I'm about to do some limited LED experimentation. Seems the best buy on LEDs that doesn't require soldering is LED xmas lights -- on Ebay, blues are about $8.50 per hundred and reds are about $10.50 (in both cases, including shipping). Probably could have gotten them cheaper in January ;). I got 1,000 blues and 100 reds, since I mainly plan to use it for spring seed starting and winter herb growing. I'm going to affix aluminum foil to an old bit of cheap wood lattice that I had in the garage (including a bit angled at the edges, forming a hood), and loosely attach the LED strands to the "hood". The whole thing will hang from the ceiling, tied with a taut-line so it can be adjusted up and down as the plants grow. Shouldn't be much work -- no soldering, no affixing each individual LED, etc. And the dangling should increase foliage penetration.

While sites promoting LED grow lights say that a 90W UFO is equivalent to a 450W HID, what I've seen suggests it's more like a 250W HID. I'll have about 75W of LEDs, but will have spent under $100 (in comparison to ~$225 retail for a UFO). That'll save me ~175W, which on 18/6 light/dark, is 3.15kWh/day, ~95kWh/mo, ~$10.50/mo. Should be a pretty quick payback time in comparison to HID. $4.50 a month in electricity costs sounds reasonable for keeping my seedlings from getting leggy and for getting me more fresh herbs in the winter.

If I remember to stop back here, I'll follow up on how it goes.