I've never used them but have a mate who swears by them, says it cuts down on watering frequency so he can leave for a few days. I am home every day to check mine so don't need them.
My light is only 250w but I have found my soil dries out even whilst i sleep.
Thanks all for quick replies ...
I might experiment, i have four seedlings all under a week old, I might try giving two some crystals and see what happens.
I noticed nappies have moisture retaining crystals inside ( i don't wear them , my daughter does ..hehe), so i suppose if it's good for a babies bum ... it shouldnt be that harmful to plants .. (female logic..)