In Nethack, a ring of regeneration causes your hit points to regenerate at a very fast rate - 1 point every turn. This is very handy in the early stages in enabling you to recover from one difficult battle before the next monster comes along. It becomes less useful later on when you heal rapidly anyway.

The ring of regeneration also causes you to use up food at 1.5 times the normal rate. This isn't normally a problem provided that you only wear it when you need to. In fact, it can be useful to wear it when you're satiated so that you are able to eat again sooner.