I have worked up a blink dog as a possible cohort for a PC in my group. Since it's an NPC, I don't have to figure an ECL, I just had to balance the HD + added character levels to the PC's leadership score. Since the base creature is 4 HD, and the PC can have a 7th level cohort, I added 3 levels of Rogue to the blink dog. It works out to be a pretty good character; blink and dimension door + finessed bite + sneak attack adds up to decent damage potential, but the hit points are mediocre at best and AC is awful.
That's okay for an NPC cohort, but a PC is going to need a better AC. I don't think an ECL of 7 is taking all the disadvantages into account. I definitely wouldn't go lower than 5, though.