Okay, this review assumes you aren't new to Skylanders, and want to expand your collection.
Double Trouble is a Magic Element shaman-like character of surprisingly good ability. I acquired him by accident in a multipack (i wanted the other two!) but he's since become quite a favourite.
His main attack is a sort of sustained beam weapon from his staff, which although relatively short-ranged, automatically locks on to nearby enemies and just keeps going until they're gone. Very useful for those powerful enemies with front shields and bruising front-attacks - you let the beam lock on and then dodge round behind them while the beam stays locked and continually deals damage. His secondary attack is to create little mini-clones of himself that explode on contact with enemies. A good one-two punch:)