I tried it on deer when I first started bow hunting. I found it to be a waste of time and effort, snake oil so to speak. Sold to new bow hunters who are still learning the art of tracking. Tracking after the shot is a critical skill that is very important. Nothing like spending hours on your hands and knees searching for blood or other sign to teach you to take your time and make a killing shot.
This was just my experience, others may have had better luck. I can tell you this, learning to track bow shot animals has led me to help recover several animals after others had given them up as either missed of not mortally wounded. While tracking a wounded bear I followed for over 1/2 a mile, he led me to the best baiting area I have ever found.
Like Steve stated, its a waste of your money. I played with them on targets and they were a pain in the butt, they can cause accuracy problems as well.
You are better off to PRACTICE, PRACTICE, PRACTICE, with your bow from elevated postions, sitting down, and be very Patient when a bear comes in and not take anything less than a good shot angle. Bears usually will give you multiple chances if you are patient.
Tracking is also a skill to work on, a good pass through shot usually gives you plenty of sign to follow, however bears have a lot of fat and it can reduce the blood loss, again be patient. I am not a big fan of expandable broadheads as I have seen way too many animals shot and a passthru was not had - that will definetly make tracking much more of a challenge. Just my opinion....
i tried this once when i was hunting whitetails back in michigan......well i forgot to take the cap off of the spool.....nice little buck came in about 20 yards.....i let the arrow fly and watch it sail straight for the heart of this young buck....then once it was about three yards away from the deer the string stopped the arrow flipped it 180 degrees and sent it flying back at me and it stuck into the tree about 4 feet below me....it stuck well enought that i had to unscrew the arrow from the broadhead....i dismantled said string tracker from my bow and haven't given it a second thought since