The bow buddy is very effective to get a more natural and loose grip on the bow. I use it for starters holding the bow too tight, which prevents them from playing properly. With the bow buddy it is very clear how to hold the bow, which makes them feel confident and play more relaxed.
Con: may cause the bow to wear out faster. The bow including the bow buddy does not fit in a regular violin case. This means that in practice you have to set it up and take it down every time the violin is transported.