We sell them at the store I work in. Frankly, other than being "cool" I don't see the need for them. If you can shoot and stay within your accuracy limits then you shouldn't miss, thus not needing to see your arrow in flight. They add unnecary weight to nock end of the arrow which can degrade the FOC some. They are battery operated which means they will wear out over time and at $10 each that gets awfully expensive.

It's usually not hard to know where your arrow goes after you release. If you are shooting with good form and following through your shot will be right where your pin was when the bow went off. In other words, the best shots you make will be the ones you never see go because you were involved in aiming and nothing but aiming.