Espeon has a really good special attack, Vaporeon is a really great physical wall, Jolteon's well....fast, and Sylveon's both good at being a special wall and a special attacker.
Umbreon does some serious work for me whenever I'm battling the other members of my school's club. It's even been the difference for my beating the prez, who made it to Nationals and onto the Pokemon.com website with his team. I didn't bring it into my battles at Regional, but that was more because my core four were so good anyway and no one expected what I was doing with them. Tee hee ~
Good thing I'm not into competitive battling then, hm? I breed out competitive battle pokemon; but I don't use em myself. I'm just a lowly collector/breeder.