In the category 'everything trying to kill you', floating monsters that look like stalactites, fill the room with magical darkness, then eat your face.
While interesting, the Darkmantle's abilities aren't that strong and in fact seem contradictory at times. Constrict and Improved Grab nudge them towards grappling, but your grapple check will suck due to small size. Similarly, flight may be strong at low levels, but you lack the ranged firepower to make use of it in combat.
Grappling is out, and so is most weapon use (Gloves of Man may work, but even then you're better off not using weapons). Casting is surprisingly viable, though.
At low levels, flight is a boon for almost any caster, your stat array supports both wisdom- and charisma-based spellcasters, small size increases your AC and to-hit with spells and Darkness is a decent buff to reduce low-level squishiness. The lost caster level hurts of course, but it's a fair trade-off for many builds.
To summarize: Darkmantles require some work to be playable, but in the right builds they can make a valuable chassis. A LA of +1 is necessary, though: its stats are a bit too good to allow as a base race.