Some people like them and every one else hates them.They do work,but they add alot of weight onto the string.
I have also heard of a guy who had to get stitches after being whacked by the string and one of those nocks.----OUCH
One real big advantage I see though is you wont have to worry about any knots coming undone.If speed isnt your thing and you have good form and not hitting your arm with the bow string periodically then go for it.