Dark energy has always been a popular energy to have in a deck since day 1 that GSC came out. The fact that you can add 10 extra damage is utterly broken, its as if your using infinite strength charms or plus powers. However it can only work on dark pokemon and you only deal the extra damage to the active. The downside of it is you can only have 4 special dark energies in the deck, otherwise the deck would have been broken.
Modified- Right now there are not very many good dark type pokemon out there but honchkrow, but electivire will electricute honchkrow. When the new weavile SW card comes out, the dark energy card will be much more useable. 8/10
Limited- Helps alot in dark type decks but useless with other types of pokemon. Hard to pull in packs. 6/10
Unlimited- Dragtrode,Neo G sneasel, Neo G Murkrow, Team Magma, Team Aqua, Tyranitar ex UF, Rock Lock, Dark Steelix. Can all abuse these dark energies and dark energy is always and will always be my favorite type of energy card. 10/10