I like the idea, since molding your Pokemon into your own personal engines of destruction is a nice touch to say the least, but I never use it. I'm a busy guy. I have a job, a girlfriend, outside obligations, a future to build, books to write, friends to visit... I really don't feel like spending all the time and effort to individually ensure each of my Pokemon only fight very specific opponents. That's essentially the opposite of relaxation and, to me at least, turns Pokemon into a chore instead of a fun game.
If you're playing competitively, I'm sure it gives you a nice edge. I never play competitively, though (personally, I don't think Pokemon's even cut out for that sort of thing, really), so I ignore EVs and IVs generally. I wouldn't mind it if they revamped the system so assigning those values wasn't so much of a headache, but I'm not really willing to put in that effort.