Since I'm now on record comparing Imp Familiar with two very heavy hitter promos (Gravikor and Ballista), I should mention that I think its major power comes against control: it acts as an action battery casting curses face down on the Warlock's own creature, then when the opponent comes into range, those curses get Enchantment Transfusioned to the opposing mage and then revealed as appropriate. If the Warlock also includes a Nullify or a Jinx in the payload, it can be very difficult to shake all the curses, and they penetrate through an opponent's nullify because of timing rules.
The Imp isn't intrinsically more dangerous than Fellella in this regard, but may be more survivable, and is more flexible because he can also cast attack spells, allows double dipping with curse ring, and the Warlock more able to put curses in his book than the Beastmaster.
For tournament play, there is also a potential issue: the imp can cast curses, but not other enchantments. A cheater might fudge this restriction.
I'm not saying the imp is OP (Wizard's Tower is stronger(!)), and Imp is vulnerable to both teleport-murder and single turn kill from focused fire. But as we get new enchantments and new ways to use them (transfusion), they seem like they're building momentum for combo book potential, and the Imp seems like a strong combo enabler that might be difficult to put back in the tube once it's printed for real.
Also, while I'm on my soap box, conjurations with ready markers should cost 4-5 more points, or come into play with their markers used