The boomeroid is a rather powerful (for an explosive) alien grenade/bomb/explosive. It doesn't pack a particularly huge punch, but is capable of softening up the tougher aliens, and cutting down shields quickly. When you first face aliens with this weapon, Anthropods especially will toss them around like confetti often causing massive damage to the building you are fighting in and occasionally killing a soldier or two. It is a step up from Marsec Proximity Mines in firepower. It is also very common, and is used enough by the aliens later on that there will often be more than enough for routine use, even after Dimension Missiles and Vortex Mines are in use by the aliens.
Overall, it is a rather dissapointing and unsatisfying weapon, causing minimal damage (again, for an explosive) against shielded humans, taking down on average a third or so of an X-com soldier's shield, and maybe half of an alien soldier's shield. It does not cause too particularly much damage unless used a lot, so it is a safe explosive to use in buildings with whom your relations with the owners are sensitive