If at that time the greatest toughness among creatures you control is 0 or less (probably because all of your other creatures were destroyed or otherwise left the battlefield in response), you won't gain any life
A deck focused on ramping up mana quickly, such as a green-based ramp deck or a landfall deck, would benefit from using Three Visits to efficiently search for and put a Forest onto the battlefield
It is a relatively interesting list as it is far more subject to opinion being that it is not just my opinion but my opinion of other people's opinions
Banlist History Restriction changes recorded for Healing Wave Generator
The difference with this card is when the card is flipped face-up by other means (card effect, attacking it), its effect will NOT activate
You can try every strategy you could ever wish, or even devise your own