Since I couldn't find anyone who had actually tried the LED light systems I see advertised, I decided to try one out. I purchased a square panel of 250 LED lights, a combination of red and blue. The fixture is about 12 inches square and about an inch thick and weighs a pound or two. Here's a picture of it.
The color of the LEDs doesn't show up well. They really are red and blue. I supported the fixture by its corners with some empty coffee cans.
To test this for starting plants, I made up two trays of small peat pots with a seed starter mix and planted small marigolds, radishes, and arugula in each tray. I put one tray under the LED lights and the other under fluorescent lights, in both cases positioning the lights 5 inches above the top of the soil. I put them on timers for 12 hours of daylight. It's taken a couple days to fiddle with this to get the two timers pretty much synchronized.
Here's a picture of the setup.
So I have a race going. I don't have a control tray under no lights, but I didn't think of this until now. I set this up on Saturday, and I can already see the radish seeds beginning to swell.