Sets are usually not very kind to the Eeveelutions. They just become subpar gimmick cards that really only serve for binder decoration. Granted, sometimes very good Eeveelution cards are released - this was especially the case in DP-block, with both the Majestic Dawn and Rising Rivals collections being extremely kind to them (with the MD-set, you could make a deck with most of the 7 all together).
After that? Not so much. You're referring to the Dark Explorers releases, correct? Let's analyze them.
Leafeon and
Glaceon here are absolutely terrible. Their damage output is so low and their effects aren't worth it at all.
Flareon's okay with a {C} Sand-Attack for 20 but beyond that it doesn't have much use.
Vaporeon's Muddy Water is very similar. (There's a Stunfisk to be released in a future set that also has Muddy Water with the same damage output, except it costs {F} and Stunfisk has 100 HP - 10 shy of Vaporeon.)
Jolteon could function decently in an Eels deck, doing a potential 60 damage for 1 Energy and fueling Dynamotor (plus free retreat helps), but the problem with that is that Eel decks have tons of possible techs that all work better. Pin Missile's okay, but we already have Zapdos.
That leaves Espeon and Umbreon.
Espeon has a rather interesting ability, preventing anything with Energy from other opponent's attack effects against them that don't involve damage, and the attack is nifty, but Espeon is mostly a tech benchsitter. Umbreon has 2 incarnations here, and both are pretty good;
the former drains opponent's HP to heal itself, and
the latter can confuse and do 20 for {C} or possibly deny the opponent's Energy completely with a larger attack.
I personally think your best bet for running one of these guys would be to use the Umbreon with Shadow Drain, and run that alongside a Dark Toolbox deck, complete with Dark Claws, Dark Patches, and the works; Shadow Drain needs very little setup, since with only one Double Colorless Energy and a Dark Claw, you are doing 50 damage and healing just as much - which means you're healing half your max HP each time you use it. And Slashing Strike is always good to go for those quick knockouts.
