DEM essentially is bonus damage for each damage tick an Energy Weapon does. The bonus damage bypasses the Deflector Shields and is applied directly to the hull. The amount of damage depends on the version of DEM (1, 2 or 3) and your Weapon Subsystem power. I haven't used it in a while, but I think the maximum damage for DEM III is around 100 pts or so. This extra amage can also be increase with Damage Resistance debuffs from Sensor Scan or Fire on my Mark.
Usefullness well in PvP. Not so much, player will out heal the extra amount of damage. In PvE, it can be very useful against target with strong shields or ones that use Shield Balancing skills. DEM combine with rapid fireing weapons like cannon with a Cannon skill running can add up to be lot of damage. Toss in Transphasic Torpedo/Mines can put a lot of hurt on targets with shield skill up.