Nick,
I think you'll find a wide variety of answers here, but, in my opinion, it all boils down to how you hunt and what you hunt. For instance, if you are a spot and stalk guy, a string tracker isn't really an option. If you're a guy who hunts from a blind/strand all the time, it wouldn't be a problem.
It also depends on what you're hunting. for instance, hunting animals that have low success rates with less due to minimal blood being lost after the shot(turkey or hog), I would be more inclined to use one of them. I have not had much luck with turkeys, but I have had a decent amount of contact with hogs. Even when you shoot them in the right place with a big rifle, it doesn't guarantee a good blood trail.
With something like a whitetail, I don't think they're as necessary, but I do believe they have a lot of added GOOD qualities about them.
Sorry for throwing the kitchen sink at you.
If you're wanting to try one out, check out chad Orde's. He makes some beauts!