I've used them a little, with mixed results. The deer I shot last fall was during the daylight, and the lighted nock didn't help at all in determining shot placement. The arrow passed through and buried under the snow, and the light didn't help me find it. On another occasion, around dusk, the arrow burried under the leaf debris and the light didn't help me find it then either.
The light can actually be a bit deceiving in determining shot placement. From above, on a pass through, the hit can look higher than it actually is. In low light conditions, I have had the nock indicate that my arrow hit brush and deflected. Probably could have determined that without the light though.
I use Nocturnals. One thing I find disconcerting is that (because the light is turned on with a plunger in the nock activated by the string, usually at release) the nock sometimes lights up as I'm drawing the arrow. This is distracting in low light situations.
My nocks are red. I think green shows up better.
I haven't noticed any difference in arrow tune. There's only a 10-15 grain difference in weight between a lighted nock and standard nock. Doesn't make enough difference for me too notice.
Regardless, I don't think I'll be using them this year.