This build is probably weak to pools faster than 9 pool, and as with most ZvZ builds, major micro/macro errors will lose you the game. Also, the build is very resource efficient in the early-mid game, so drone scouting isn't something I'd suggest unless you tested for the ideal time to do it (I haven't). Maps with ramps and no more than 4 bases (2 base is ideal because of lack of drone scout) are best for this build, imo. Anyway, here it is:
9 Ovie (scout after its done)
12 Pool
11 Gas (directly after pool)
12 Hatchery (at natural)
11 2 zergling eggs (rally to ramp)
13 zergling speed; 2 zergling eggs
15 lair; 1 zergling egg; take 2 drones off gas (don't take the drones off in anticipation of 100 gas, do it after you make lair. If you take the drones off in anticipation, your spire will be late)
16 overlord
From here you mass zerglings, making drones only when you can't pressure and don't need more lings for defense (often I don't make any extra drones until after spire completes, though that may be a failing of mine). I think you should always attempt to pressure with lings if your opponent has taken his nat. If you opponent defends well, attempt to run past his lings into his main's drones. Cause havoc. As soon as you place your spire, put 2 drones back on gas, and if you've made a few extra drones, make another gas at your nat. Once your spire is complete, judge what your opponent's ling count looks like. If he has a greater ling count, you should probably build muta, if your opponent has a similar ling count or muta already, you should probably spend all your gas on scourge to take out overlords and any muta your opponent might be making/have. From here go into normal ZvZ, produce as many muta as you can and make lings/drones with excess minerals.
Disclaimer: I'm not a good player, and as I said I haven't used this build vs solid zergs. None of my games with this build have lasted longer than 7 minutes, so I'm not sure how it performs if you're forced into the zerg late-game. I'm not advocating this as the best ZvZ build order by any means, I've just noticed its been working for me so far... I'm sure it has weaknesses I haven't had the chance to experience/identify.
Replay: Here's a game of me using the build order vs a friend of mine. He lost our first two zvz's very quickly and I told him my build order for this game, which he used (though his ling timing was a bit different, and he made lair before speed, I believe). So we're using close to identical build orders here-- I know, probably not the best example of how the build works, but this is among the longest zvz I've had with this build. Note: I miss my overlord in this game and "make up for it" by making way too many... those minerals should've been going toward zerglings.
Gosu Replay: Here's the game I stole the build order from. It's sAviOr vs Sen game 1 on Blitz X. I think I read somewhere that people thought sen had the superior build order, and they're probably right. Sen had more than enough lings to defend, and of course he had more gas (his drones stayed on gas the whole game), but if sAviOr's build choice puts him behind, he never looks very far behind and because of the faster pool combined with the many lings and the fact that this build provides and a spire that is about as fast as Sen's, he's constantly threatening to do enough damage to even the odds.... And yeah, sAviOr's micro was godly, so that helps, hehe.
Hope someone finds this handy!