This spell can be used to save a life when low on health and under attack. It is commonly used to get an early first blood by healing oneself for more than one's opponent expects. It is also a skill used for "baiting", where you make your champion appear weaker than it actually is, to lure your enemies out of position or to get them to tower dive you. A similar baiting tactic is also commonly used with Barrier.
This summoner spell is considered by many to be vastly superior to Barrier for a number of reasons - the heal is permanent, affects two people instead of one, and gives a 30% speed boost to yourself and your ally. However, Barrier is not affected by Grievous Wounds, has a shorter cooldown, blocks more damage (against a single target), and can be used while you're still at full health (good against 100-to-0 burst).
Healed champions are debuffed for 35 seconds, causing subsequent Heal casts on them to be 50% less effective for that period.
Heal is affected by any form of personal health gain modifier on yourself (but only yourself), including:
A debuff is now placed on champions healed by Summoner Heal for 25 seconds. Subsequent Summoner Heals will have their effect diminished by 50% on allies with the debuff. Your Champion will still receive the full amount of the heal if cast by your Summoner.
Mender's Faith (mastery) now provides 10 × level bonus health on Summoner Heal from 75 flat bonus health and 10 seconds cooldown reduction.