I've heard nothing but good things about them. They really don't affect arrow flight, but do allow you to follow your arrow to ascertain shot placement and aid in retrieving your arrow after the shot.
I believe Kell is right as far as the record books go. There are stipulations about no electronic devices and whatnot. I'm not sure which ones would be affected, but I have heard that.
Some states/counties may also have laws against electronic aides when hunting, so be sure to check your local regs (like kell said)
"When a hunter is in a tree stand with high moral values and with the proper hunting ethics and richer for the experience, that hunter is 20 feet closer to God." ~Fred Bear