ITT everyone spends too much time mini-modding.
Yes, it has been done before, but it turns out it's not much of a success most of the time in OU. You just play to their strengths; not saying much since only 3 of their base stats are good.
3? IMO Flareon and Glaceon are the only ones who completely suffer because of stats or movepool or what have you. Glaceon, for me because the speed is just not enough to work with, but in no way is it a dealbreaker. Flareon for obvious reasons fails pretty hard.
Vaporeon and Umbreon both make good sponges/walls, and Vaporeon can hit back pretty well. I'm still shocked sometimes as how well Umbreon can take a hit, even with all the additions in D/P that set him back.
Jolteon and Espeon are good special attackers with good speed that have been consistently good, Leafeon has the potential to be a good Physical attacker and I've seen him do pretty well and know people have had great sucess with him.
The one thing that I feel hinders an eevee-lution team is that they have movepools that are left wanting. Some might say that Espeon's typing hinders him, but I've had success with Hypno, Alakazam, and Gardevoir in OU.
Flareon is the only one who I feel is almost worthless, but you could always run it modest and use it as a Special attacker. If Glaceon and Flareon could trade stats with eachother, I think they'd both be a lot better. Even with 65 base speed.
5 out of 7 who could do well in the right hands isn't bad. It's a gimmick team, you can't expect too much, but I think people underestimate a lot of the eeveelutions.