i use 3 of the blue ones over my frag tank. i bought one powered unit and two slaves. for me they do wonders when i turn them on. again, though, i ONLY turn them on when i want to take a good look at things and for pictures. i believe you see the full potential of the corals with them running.
the frag tank is 24x24x8. i run four t5s over it mounted 24" up and they are not overdriven. i also have 2 more mounted with them and turn them on also to take a good look and for pictures. they, too, are not overdriven. the bulb combo is very blue. when they are all on it is 3 ati blue+, a kz fiji purple, and 2 uvl super acts. when i turn the leds on with them there is a brilliance that just isn't there w/ only the t5s. it is really a dramatic change. my favorite view is leds only, though! :D