I have 3 of their fixtures. My first was their custom reef spectrum, second was just a deep blue / 6500K white and third was a modification of their custom reef to spectrum to add UV and cyan.
THe unit construction is awesome, I mean really really awesome. I got mine with dimmers to sit on top of a glass lid over my 20 long. Way way way too much light for my tank, so I ended up dimming them down to 25% - 50% :)
The only negative I see has nothing to do with BuildMyLED and I want to make that clear. These are great lights.
Over my 20 long, sitting on a glass lid with just 13 inches between the lights and the sand, the disco effect is really really bad. I'm betting this would not be noticeable if they were hanging a decent height above the tank but I don't have that option. Because of this I ended up placing my blue/white fixture on the back half (disco effect negligible using just 2 colors of LED's) and used a normal output t5 fixture under driving HO t5 ATI bulbs on the front to get the blend of colors I like.
IMO if you are going to hang these above your tank I think you'll be very pleased with how they look and perform. Be sure to check out their forums for other examples of how people have used these over their reef tanks.