I use them in every ball but my spare ball. Some of the things I like most about them is that I can switch balls and have the exact same feel in the thumb, and I can change thumb hole sizes by switching out the slug. This is great because my thumb changes size quite a bit based on the outside temperature and how many games I've bowled. I've never had a problem with them getting "loose", but a friend of mine added a thin sliver of tape to his to "shim" it snug. He's used them for quite a while and it seems his slug was getting a bit worn. The ones I use are called Switch Grips.
