I am using cree 3 watt leds (24 of them) to supplement my T5 lighting. When I switched from metal halide I found that I missed the shimmer, so I flanked the T5s with the crees. LEDs are great because you can dim them, put optics on them, and really tweak the color/spread of them. They are really new though... and many people who are using them as a sole method of lighting are finding some colors are easy to keep, and some others not so much. They believe that this is because of the very narrow spectrum these bulbs produce. The newer fixtures being produced are using both the royal and standard blue leds AND the warm white and cool white leds. These seem to be able to produce more colors. There are some great threads over on RC about LEDs in the DIY section if you have a few months to read them