I just unlocked this skill which feeds on the life force of my enemies to increase my Intelligence by 30 seconds. When I activate it, a circle of light surrounds me. What's not clear to me is whether enemies need to be in this radius of light for the Soul Harvest to take effect. Or do I activate the skill and then kill my enemies to reap the effect's benefits and how can I tell if I am benefiting from it?
As someone who has only played Witch Doctor and uses Soul Harvest as a main part of my strategy, I can confirm that the monsters need to be within the radius upon casting.
Note that the max for this effect is 5, which is not hard to achieve. Also worth noting: it is very effective to charge headfirst into a cluster of enemies, harvest their souls, then use Spirit Walk to find your best point to attack from.
I play Witch Doctor, and soul harvest works as the first reason you posted, enemies need to be in the ring that you cast for the intelligence gain (up to 5 stacks, 1 per enemy harvested), as well as the added rune you added to your soul harvest.
Life Siphon rune is also the best rune in my opinion to add onto that spell once you unlock it. Its pretty much a full heal every 6 seconds if you get the maximum amount of enemies in your soul harvest. Its a really good spell :D
As a small additional note, the duration of the Soul Harvest effect is longer than the cool down on the ability. This means that it is possible to harvest again, before the first effect wears off. If you do, the new harvest overwrites the previous. So it is possible to say have 5 stacks, encounter a lone enemy, and harvest down to 1 stack (which is to your disadvantage).