We have one for my DS, it works like a dream now (he's 3 and a half). It didn't work so well when we tried it when he was younger (about 2) but he was a terrible sleeper then and tbh I probably didn't enforce it enough because it was easier to bring him in my bed!
DS knows he has to stay in bed until the sun comes (we've got it set for 6am at the moment), sometimes he'll sleep past it but sometimes I hear him awake at 5.30ish but he doesn't come in our room until bang on 6! I'm going to try slowly sneaking his round to 7 because 6 is a little early for my liking!
Xxx