I've always wondered if I was missing the boat by not using water crystals, also known as water retaining crystals or hydrogels, in my container gardens. Lots of people recommend them and claim that if they mix water crystals in with their potting soil, they have to water much less often. Water crystals are also often advertised as being "green," because they supposedly conserve water.
There are two types of water crystals, those made that are starch based (Quench) and the more common type which are made from polyacrylamide (Soil Moist and WaterCrystals).
There is disagreement as to the environmental impact and toxicity of polyacrylamide. But some studies, suggest that polyacrylamide, which most water crystals are made of, is a carcinogen.
While the jury is still out on the safety of the polyacrylamide water crystals, the question may actually be moot because it appears that they don't work particularly well anyhow. According to Fine Gardening Magazine neither kind of water crystal makes an appreciable difference in the amount of water your soil will hold.
Studies run by Jeff Gillman, author of the Fine Gardening piece, as well as the book, "The Truth about Garden Remedies," showed that the starch-based hyrdogels provided some benefit in prolonging water retention in container gardens, but only "an extra day at most between waterings." But in his studies, the "plants potted with polyacrylamide gels did not fare better than plants without."
So I think the bottom line is, save your money and get out your hose.