I swear by them, that's the whole idea. There are many brands out there, but they most definately can help hold the moisture in the soil. You have to experiment so that you don't keep things too wet. The links below might help.
Kell I agree with Azalea. The first time I used it only on my tomatoes. When I transplanted them in the garden the soil in the pot was bone dry and the roots were moist. I use it in all my pots and my tomatoes, hydrangeas and clematis.
LOL! I usually add the crystals to a glass of water and let them rehydrate before putting them in my pots; it seems to cut down on the Jello Syndrome. Been there, done that, too!! The kind I use right now is Soil Moist. You can re-use them, so save that potting soil from year to year! :)
I did not like those crystals, for one thing you have to bury them dig or they will come to the surface and become very slick, in fact you could fall down if you slip on them. I don't think they really save water and they don't last that long either. Also because they rise to the surface they pushed some seedlings right out of the ground. I think old fashioned perlite and vermiculite work best. Home composting over all will improve the soil to the point that it stay wet longer. A layer of homemade compost cuts down on watering better then any thing else. But, any type of mulch will also help.