You recharge and lose 2 turns. In Lilycove May's Wailord used rest 12 times and my Pikachu used 2 electro balls and guess who won. Me! Well I did use 2 other Pokemon which did nothing.
Speaking from experience and logic, Rest is never worth it in-game unless you're holding a Lum or Chesto Berry. You're better off using a Hyper Potion or Full Restore. Rest leaves you vulnerable for multiple turns while a healing item only leaves you vulnerable for one.
I think it's decent with Sleep Talk Snorlax. In the singleplayer game you would be better off using items as Doctor Disco said earlier.
It can be worth it with a rest based move set. For ex- I have a wiscash who will use earthquake, rest use sleep talk or snore for extra damage and rince and reapeat