The power is a really good example of really bad design.
In the hands of a competent player, the power does:
A little bit more damage that the other warlock 29 dailies, assuming it gets all 4 allies (on the order of about 50 damage or so)
In exchange for
40 points of damage to the party
Yeah, good trade there.
Furthermore, if it misses, it STILL does 40 damage to the party, and the damage done by it to foes drops to 40
This all assumes you're a dark pact warlock - otherwise it's much, much worse.
Further, any effort by you to increase the damage it does to make it a meaningful power will likely be thwarted by an explicit rule in the DMG.
So - if you've got a good DM who follows the rules, the power sucks, and most likely (and also with good reason) pisses off your fellow players (mostly because YOU don't take the damage - only them).
So, in the scenario where you've got a bad DM? A player still has to go to a lot of effort to exploit the power (and by exploit, read "make it worth casting"). It's not easy for a player to run around with a lot of allies.
A DM on the other hand?
Hmm, one warlock of [level+2] and 30 minions of [level-2] should do it.
And do you think this style of DM is likely to ban the power?
So, I vote the power as "worst power we've seen to date". It's a waste of a daily, a waste of your turn, will piss off your fellow players, and is eminently abuseable by DMs and only by DMs. I award it no points, and may god have mercy on it's soul.