Unown E is kind of nifty. 50 HP is lowish, Weakness to Psychic is fine seeing as how Psychic Pokemon are not being played much, and 1 Retreat Cost. Its one attack, Hidden Power, is overpriced at 2 Energies for only treating your opponent's flips as Tails. What makes Unow E ok though is his Poke-Power, EQUIP, which allows you to attach him as a Tool to one of your Pokemon for +10 HP. Granted, it's not as powerful (or easy to use) as Energy Root from last season, but an additional 10 HP could potentially let you live through an attack. Of course, there are other (and somewhat better options) such as Buffer Piece or a Potion to heal damage.
Modified - 4/10 (Ok, but not exactly going to be used en-mass)
Limited - 8/10 (I found it useful here, as the 10 extra HP is useful)
Unlimited - 1/10 (Base Set Gastly is better)