I, too, use and prefer them. Mine has room for a few pieces of tape out of preference. The initial silicone mold is basically the only expense (minimal) and after that pouring the thumb is very inexpensive.
Too many of the changeable inserts have broken and rendered the ball useless until the repair can be done. They are "cool" though because I see guys playing with them all night long... Inserts break at their base, the part that they connect to in the bottom of the thumbs break, inserts fall out into the machinery, bowler can't get his insert out of ball, bowler can't find his insert he used last week because it's in his other bowling bag, the list goes on... The idea sounds great, but it's implementation is imperfect by far depending on your pro shop's expertise, etc. Some methods are being improved upon but...
