So now that Druids can wildshape into an Intellect Devourer, Great Old Warlocks with the Pact Of The Chain should have an option to have the Intellect Devourer, Us, as their familiar.
Even if they do, Us still wouldn't be as good as the imp. The imp can fly and so covers far more ground, it can turn invisible at will and so is a sneak attack monster, and it hits harder.
Us just has more hp and would be a better damage soak, other than that, the imp wins. I would still choose the imp every time.
