Play sand has all kinds of crap in it. It's not washed, has bits of organics in it, and is not sourced in any particular way, so what you get in one bag can be quite a bit different in another. It is also not sieved to consistent grain size, so you get dust and pebble-sized pieces in it. That's ok, but the dust part can be irritating. And lastly, the light color shows off every piece of poop and mulm.
$8 for 50 lbs is a bargain compared to the prices some people are paying for substrate. Price Aquasoil sometime!