I use the brand, Tracer nock and I am very happy with them.
The batteries last a long time, are replacable and reasonable in price. I bought backup batteries at the local Battery Plus store.
The hunter model burns for about 11 hours continious if you do not shut it off with the magnet and the practice model I believe stays lite for 15 seconds after first illuminated.
Any added weight to the arrow will effect arrow flight but once installed and sighted in they fly true just like any other nock. Just remember if you use a light weight broadhead, you want to keep the arrow forward weighted. If you increase the weight at the nock and arrow fight changes a little you may want to increase broad head weight to maintain that forward weighting.

I took my ID elk with them two years ago and they work fantastic.
I have also taken whitetail and inboth low light and mid day they make tracing the arrow flight unbelievable.

And yes they are just plian cool to watch in flight.

I e-mailed the gentleman in TX the distributes them and advised him of this thread. Maybe he'll log on and enlighten us a bit more