This item is amazing! My son is almost 16 weeks old and we've been averaging 3 hours sleep a night since he was born because he won't sleep on his back, only on his side, but keeps sliding onto his back and then complaining until he's rolled back.
It's designed (officially) to keep babies on their backs, but because the width between the two sides can be adjusted, you can use it to keep a side-sleeping baby in the position he likes best as well.
He now sleeps soundly from around 10pm to 6am, only waking if something's wrong (dirty nappy or the like), instead of waking every 15 minutes because he's on his back again. It's not an exaggeration to say this thing has saved my sanity!
My health visitor wouldn't approve of it, but she's not the one going demented on 3 hours sleep a night.....