1. It is not recommended for normal gameplay unless you've got a powerful system. It can be quite taxing. Also, during normal gameplay you're probably not going to have your camera in your sims' faces the whole time, so you'll be unlikely to notice the benefits.
2. Not in game. You turn it 'off' by substituting back in your original graphics.sgr file.
3. It depends what your default settings are. You need to turn your sim detail and texture detail settings up to the highest in order to take advantage of the mod.
4. If you use custom content that's been designed to take advantage HQ, understand how to light your sims correctly, and zoom in very close, it does make a difference.
5. See 1
6. I'll leave this for the more computer-savvy people to answer, but I will say that the mod can be taxing on your graphics card if it's a lower end one.