finally someone else talking about LEDs. here's my experience with them:
1st crop
before I bought my 120w LED (Red, Orange, Blue and White LEDs), I was finishing off some outdoor girls under 600w HPS so I stuck my new seedlings under there too. After the girls where done, I ordered the LED and left the new seedlings on a window sill (not a great idea). The LED arrived and it had a faulty plug and I also had guests, so i stupidly delayed fixing the plug for about a week. After that, I set up the LED and grew the new girls (Fast Bud) to harvest. I don't know if the early mess up affected the overall growth (probably), but the final results were a flowering period longer than expected (at least 3 weeks) and buds that were fragrant and frosty but not very dense. In the grow tent, I also put an almost dead chilli plant that started to get nice and vigorous again.
2nd crop
Because of all that, I recently started a new crop with the hope of avoiding the previous mistakes. I planted 4 other autoflowering beans, La Diva, Big Bud, AK47 and Master Kush, and tried again. To my huge surprise, it didn't work
The seedlings, after a promising start, started to yellow and curl and pretty much stopped growing. In the meantime, the chilli plant in the tent is doing great, flowering and producing lots of chilli peppers. So, what the fuck???
My conclusions (and those of a friend of mine who had the exact same phenomenon happening) are that LEDs are good on reasonably established plants but not so great on seedlings. The plan is now to use a 120w CFL (Red and Blue) for a while and then add the LED to contribute. We're also contemplating the idea of finishing the plants off under the expensive 600w HPS.
I hope all this shit makes some sense