I've done it about 15+ times (7 chars with 2 to 3 times each) and have never failed. One time, we even did it with just a tank and a controller, using 2 accounts to get the mandatory 4 then dropping it.
From my experience, biggest failures are when there is such a wide range in the levels of the players. The lower levels being rather useless when the much higher level monsters can't be hit/affected.
I've been in a group where a tank was complaining about not being able to hold aggro, but was unwilling to be sidekicked because they would get less xp. Told them goodbye and left the TF because all it takes is just one player who is unwilling to act as a team player to cause problems in the future and it just wasn't worth it.
