The only regenerating health system I've ever liked is FarCry 2's. In that you can 'regenerate' the last 2 bars of health if you have enough time/cover and you are under a time limit because at that point you are bleeding pretty severely. This system is made more tolerable by the actions that go along with stopping the bleeding, while unrealistic they are more immersive than most systems out there. (For those that don't know, when you attempt to regenerate those last 2 bars, your character will do things such as tear bullets out of him, crack bones back in place, put out fires on his clothes, pull metal out of his ribs, every action is dependent on what set him under 1 health bar as well.)
I find systems where you can regenerate to full life to be retarded. It eliminates a number of possible scenarios and dulls the players wits as they choose certain methods of attack based on knowing they can regain the lost HP. In other words, there is no slow death, only quick death, the slow death has been eliminated because it was too difficult to manage for certain players.