Usually when the first three arrive, they don't have a sentry yet (The first three are about 15 to 20 seconds ahead of the next 4). If they do have a sentry, and they block, then you fall back and then attack with all 7. If they don't use it, you can usually use those three to start killing their units, and the next 4 come in and finish the job. If they are rushing an immortal (high level players often do this vs Zerg) then you can usually snipe the pylon. If there is more than one pylon powering the building, then it's tougher.

In 1v1, a variation on this strat is to go all-in - send your queen after you spawn larvae, and then when the second 4 come out send most of your drones. With 7 roaches + drones + queen, it doesn't matter if he gets out the Immortal, or infact what he goes for. Only thing I've seen is a hard cannon wall, which then commits him to a single base Void rush - which is easily countered.

So yes, this strategy is very unbalanced. Played 22 games tonight, 7 losses. 2 of those losses where on 2v2. Last 10 games I played? 9 wins, 1 loss.

I hope Blizzard fixes this soon :( it's going to make Terran and Toss very susceptable early game.