Let's be clear, this stuff is a a little messy and a lot weird. If you're the kind of person who thinks it might be fun to squish around in a bathtub full of jello, then this is the stuff for you. If you think that sounds like a disgusting mess, then you surely already know to stay away from something like this, right?
My 3 year-old (and her 35 year-old dad) had a blast with this stuff. It is messy and gooey and strange, but that's sort of the point. It seems like it's easy on the skin, and didn't stain or even leave a residue in our tub.
A couple things:
I found this stuff, same size box, etc. at Target for about a third of this price ($8.99) in March of 2012. (It's near the Silly Putty, Play Doh, Moon Mud, Floam, etc.)
Fill the tub with maybe four inches of water, add the powder and the kid, and give it a couple minutes, then add more water to get it to the consistency you want.
Sprinkle the powder carefully and evenly. If you get too much in one spot, you'll get lumpy goo.
The goo did take a while to turn completely back to water, but it wasn't a big deal. I used the powder and some extra salt (which is all the powder is), and it mostly thinned out in a few minutes. It might have continued thinning, but I didn't bother waiting. I just turned on the water to dilute the remaining goo, and sent it down the drain. No problem.
Bottom line, this stuff is fun.