The first seemingly broken build. I played 5 games vs. Protoss, and 3 of them involved me losing to this build (as the Zerg). (FYI: one of the others was against some guy who didn't know what he was doing, and the other was against a guy who contained me with cannons because I wasn't paying attention and exhausted me).
I'm far from convinced that it is invincible, but it is very tough. 4-pool back when Spawning Pools cost only 150 minerals tough.
One of the problems I ran into was even finding out if it was happening. Granted, I didn't Drone-scout, but that may be the only way to know what's up.
The other problem is the counter. Roaches and Banelings, either of which can probably do a good job, cost a sizable quantity of gas.
There was this one game where I almost beat the guy who went for this. It was on Lost Temple. I went for a super, super fast gas. Extractor before Overlord, fast. But I only put two Drones on. That gave me enough surplus gas by the time the Baneling Nest came out that I could do something. You also get early speed for the Zerglings.
I fought off his early attack (Spawn Larva was utterly vital to keeping up with his output). He tried to set up some Proxy Pylon BS, but I shut that down quickly enough. The problem was that I moved out with my SpeedLings and morphed Banelings a little outside of his base. I did it in a spot he wasn't watching, so he didn't catch me with half my army down. The problem was that, while my Banelings were Morphing, he sent way too many Zealots around in a direction I wasn't watching. My army just couldn't get back in time.
Looking back on it, I almost certainly should have used my advantage to expand, with Zerglings and Banelings keeping those damn Zealots out. Alternatively, I could have gone for Roaches; those might have worked better defensively.
I can tell you this much: don't fast expand against Protoss unless you're sure he's not going 3-Gate Zealot.

