It can be a positive Elixir trade for the Poison card if your opponent places it down. It nullifies the Poison's effects for the most part as the healing per second of the Heal is greater than the damage per second of the Poison, but at equivalent levels the Poison will do more total damage than the total healing capability of a single Heal, because the duration of the Heal is much shorter than that of the Poison's.
While the Heal has healing capabilities, it does not heal any troops that have been destroyed. In fact, if the troop is eliminated by a particular attack in one hit, the Heal will be incapable of doing anything. For example, you cannot keep Skeletons that are under an enemy Poison alive using a Heal unless the Skeletons are sufficiently leveled such that they do not die in one hit.
Please Note: This card's statistics may change before its launch.