The shield changes colors (from blue to yellow) right before it explodes. As an enemy, you can use this visual indication to avoid the explosion.
You can save the four charges and kill four or more minions, and heal nearby allied champions for around (50+20) per charge. This mass heal can help tremendously in a teamfight.
On Summoner's Rift, assuming Face of the Mountain's user kills the most valuable minions available, it provides:
13.125 per 10 seconds to the user, while also sharing that same gold with the lane partner.
Between 20 and 35 minutes, this is increased to 15.83 per 10.
After 35 minutes, this is further increased to 22.5 per 10.
Assuming Face of the Mountain's user kills one minion every 20 seconds, it provides at least 12.5 health regen per 5 seconds to the user and to the lane partner, not taking into account the missing health ratio.
Reward:Shield Battery, a permanent shield that regenerates slowly outside of combat.
Shield Battery: Grants a permanent shield for 255 − 340 (based on level). The shield regenerates slowly when out-of-combat. Executing minions with Spoils of War regenerates 64 − 85 (based on level) shield value.
Unique Passive – Spoils of War: Basic attacks execute minions below 200 health. Killing a minion heals the nearest allied champion for 2% of your maximum health and grants them gold equal to the kill plus 10. This effect requires a nearby allied champion to trigger. Recharges every 30 seconds, up to a maximum of 4 charges.
Unique Passive – Gold Income: You may only be in possession of 1 Gold Income item.
Unique Active – Deadly Phalanx: Consumes 20% of your current Health to shield target ally for 10% of your maximum Health for 4 seconds. After 4 seconds, an explosion originates from the target dealing 10% of your maximum health as magic damage in an area. The shield does not need to persist for the explosion to occur (60 second cooldown).