Considering that you can activate his ability as many times as you have green mana to pay, this is a pretty powerful card. It's especially good if your opponent isn't playing any green mana sources. On top of that, it's a 3/3 flyer for 2GG.
It's not hard to see why this is on the Reserve List, as it's actually quite powerful on paper. In practice, the Efreet doesn't really see much play anymore, but pops up in Cubes and Commander decks, here and there.