I've used NVGs quite often, and the rendition of them in DCS is eerily accurrate. They help quite a lot but certainly don't turn night into day. And yes, you feel badly blinkered and have no real depth perception.
I should qualify that by saying the NVGs I use are probably on the crappy side, but maybe they're all pretty similar. The portrayal in DCS is the most accurate reproduction of my experience I've seen in a computer game.