It has a time and a place. Slightly higher passive lifesteal, with an active ability that gives you +100 attack speed (the highest single boost in the game) and +30% movespeed for a respectable period of time. Plus, it's cheap. 1900 gold total. Helm of the Dominator is 1850.
The downside is +30% increased damage from all sources (except HP removal) for the same period. Generally this means you need to either be capable of withstanding said damage with a lot of HP or damage reductions, you need to avoid the damage altogether (Void's chronosphere, Sniper's range, or just using it while advantageous), or it needs to be to your advantage.
An example of that last part is Blade Mail or Spectre's Dispersal. Since they act off of actual damage dealt, they will return just as much increased damage for the effect of MoM.
As mentioned, it can be nice in some situations. It's highly situational though, as it does make you exceptionally squishy. I don't like taking it on squishy characters like Slark (even though he does recognize it with a custom speech line, and needs as much early attack speed as he can get) anymore, but on something like the aforementioned Void, a built-to-last Alchemist or even a beefy Spectre to get the most out of her chasing and desolation? Sure, if it fits.