Indeed, Bitshares has launched an exchange that's decentralized. I have been trading on there for about one week now. I have seen some good deals in the market, and have turned a decent profit, exchanging bitshares for BitUSD (a pegged asset) while the price of btc fell from 260 to 230.
Also and probably most awesome is that BitUSD and all other pegged assets pay you %'s just for holding them!
Have questions, you wil be welcomed over on their own forum. On here i'll answer questions to the best of my ability.
Matthew