I bought this set of 4 (flexible silicon) ice ball molds for my husband / bartender-extraordinaire. They work great. The molds come in 2 pieces each which fit together snugly. It takes a little bit of tugging on the silicon mold to get the ball of ice out once its frozen, but I haven't had any problems. I didn't need to run them under water, either, which I didn't really want to do, so was glad to see I could pull the mold pieces off the ice w/o that. There is a fill line marked on each an a little hole at the top to allow for ice to overflow. I found that the fill line is pretty accurate and have been able to make pretty consistent round balls. The bottom part of the mold has a little flat area so that it can sit flat & level in the freezer (the flat area is only on the outside, the inside of the mold is still round).
I like that they are 4 separate molds, instead of one big one connected together, which allows me to tuck them individually in small open areas in my small freezer.
I freeze up a batch, pop them out, then put the balls in a ziploc for use later, and then can make another batch.