Generation II onward
Softboiled restores up to 50% of the user's current HP. Softboiled will fail only when the difference between the user's current and maximum HP is 0.
Outside of battle
Similar to Milk Drink, Softboiled can also be used outside of battle to take away some HP from the user and restore another party Pokémon's HP.
Yes, it heals your Pokemon. During battle, it only will heal you, but outside of battle, it will heal your party members too.
and also Experience