I don't think it's OP though. There are several abilities that are easy to use and provide amazing results. Blind comes to mind - it's cheap and easy to use, and blinded enemies just sit there like idiots. Sure, they could run around or try to attack, but they don't. It's low level, so by the time you are high level you are crushing the saving throws of most mobs and you can cast it every round. I've blinded all the bosses, even the final boss, and they just sit there while you destroy them.
Blind really is amazing, and is probably broken right now because enemies should not just be standing around while blind. The fact that it works on bosses really allows it to shine. But pound for pound, Rain still outperforms it. Stunning 3-5 guys for 8 AP is some of the most efficient CC in the game, and only gets better if you have multiple people in the party who are running Rain and can make the puddle larger and larger.
But would not you need to get the lighting rod talent to avoid being stun, unless you are running a full ranger party. I find stunning to be particularly annoying and to often have collateral effect.
You can get Lightning Rod as early as Level 11 with a normal character, and Level 7 as a Lone Wolf, and it really is as powerful as it seems. It is one of those talents that makes an enormous difference something I recommend even for melee characters. A 2 Handed Warrior with Aerotheurge 5 and Man-At-Arms 5 is the most powerful character in the game.
Until level 11/7 your melee characters should focus on murdering people outside the electric puddles. Rogues have the advantage of swapping from daggers to bows after all the non-stunned people are dead.
If you have lighting rod, do you run lighting strike and are able to lock down enemies.
Lightning Rod + Air Resistance lets you run lightning strike with impunity, throw around chain lightnings like nothing, get full effectiveness out of storms, and even stand in static clouds during combat. Out of combat you instantly disperse static clouds which can allow your party members to pass safely through them.