You can see in the picture that looking through the sheet is kind of like looking through plastic. Because the structure of the sheet is a series of vertical rows, the walls of those rows breaks up the image you see. You can see general shapes and things outside of the greenhouse, but it would be difficult to make out any detail.
In terms of light, it does reduce the PAR in the greenhouse significantly. I would guess we are losing about 20% (just a guess), but what is getting through is still very intense. It is intense enough that I am already planning on building shade cloth frames for the tanks. I can tell that on a sunny day, direct sunlight will probably cause mass bleaching.
The sheets are designed to be decent insulators because of the air pocket in the sheets. We have not gone through a winter yet with the greenhouse fully sealed and the gas heater on. Even in the partially constructed state however, it was always about 10 degrees warmer in the greenhouse than outside. I think once we winterize all the doors and vents as well as fire up the heater, the greenhouse will be able to maintain heat pretty well. Having a 2500 gallon heat sink doesn't hurt either. Water temperatures remain remarkably stable when the air temperature is stable.