Very good skill for single target tanking. The tanking rotation generating the most threat is: Mangle as often as possible; while Mangle is on cooldown, use Lacerate; if you still have excess rage, use Maul in parallel.
When tanking multiple mobs at once, Swipe (Bear) is usually a better choice than Lacerate.
0600Patch 6.0.2 (14-October-2014): Lacerate is now available only to Guardian Druids.; Lacerate now generates 2 Rage, has no cooldown (down from a 3-second cooldown), and the periodic bleed effect triggers every 1 second (down from 3 seconds).
0400Patch 4.2.0 (2011-06-28): Lacerate initial base damage has been increased to 3608, up from 2089. Initial damage attack power scaling has been decreased to 5.52%, down from 7.66%. Periodic base damage has been increased to 69, up from 16. Periodic damage attack power scaling has been decreased to 0.369%, down from 0.512%.
0400Patch 4.1.0 (26-Apr-2011): Lacerate bonus threat has been removed and replaced with increased initial damage done.
0400Patch 4.0.6 (8-Feb-2011): Lacerate damage has been reduced by approximately 20%.
0200Patch 2.4.0 (25-Mar-2008): This ability now deals additional damage based on the attack power of the Druid. Introduced attack power scaling: Every 20 Attack Power adds 1 damage, per stack, over 15 seconds. Mangle further enhances this value.
0200Patch 2.1.0 (22-May-2007): The tooltip has been adjusted to indicate it does initial bleed damage when the ability first lands.