Originally Posted by lunatech
... As far as heat, the ones pictured look like a window and has a pleated shade you can close...
I've read on other forums where people have put covers on skylights and the heat trapped between the cover and skylight caused premature damage to the plastic of the skylight. The same for when people try to put bubble foil on the inside of dual pane windows. The reflected heat would damage the windows (usually the seals but sometimes the glass would crack).
Keep in mind I live in an area infamous for the amount of sun it receives. UV is brutal on plastics (and mostly everything else) and the heat gain through one would be significant (you could cook under a skylight here; heck, when I was a kid, I put a cast iron skillet on the driveway one summer and cooked an egg that way). Heat gain may not be such a problem in your neck of the woods. I would still worry about leaks, though.