I've known people to use about 2 inches of play sand as their substrate with no problems, but I've never used it by itself...
Personally, I use soil substrates with about an inch (maybe a little less) of sand on top... I've used play sand and "reef sand" before with no problems. (Brand is below- be careful, some reef sand is extremely alkaline and designed for marine tanks, but strangely this ine isn't.
It's a little bit more expensive than play sand, but still cheap- $4 for about 5 pounds, and it has a large (for sand) grain size. Also, requires no rinsing, and available in tons of colors.
Play sand's main drawback (in my opinion) is the color.... Usually whitish, and white substrates stress out fish.
If you use sand or sand/soil, you'll need to make sure the sand doesn't compact. MTS snails will help with this, but you need to occasionally stir the bed (if sand) or poke it (if you have soil underneath).
For pictures of my black sand/soil tank, check out the link in my sig to my soil substrate topic.
For pictures of play sand, look up some of angel079's tanks.
I admit, I'm behind on my maintenance. Since that pic, I've cleaned it up a lot, but haven't been able to borrow a camera.