Quoting from bartender Jeffrey Morgenthaler's recent book:

> My experience using these has resulted in more than a few cracked ice balls, whose deep fissures essentially increase the surface area of the ice ball, negating any benefits in that department.

I had pretty much the same experience with my molds (I was using the Tovolo brand, rather than the one you linked to, though).

They work better with filtered and double/triple boiled water.

Or you could go for a pricier option such as http://www.wintersmiths.com/ (which uses directional freezing).