Mt. Gox might be the most popular one, but unless you're going to buy/sell an extremly high amount of Bitcoins, other exchanges are viable too.
Currently I like Bitmarket.eu most: This site acts as an escrow just for the Bitcoin side of the trade - the "real" money is directly transfered between buyer and seller (via bank transfer or Paypal). That's somewhat more distributed than other sites that act as "full gateways" between buyers and sellers.
Numbers are of the essence. That site has less than 1% of the volume of Mt. Gox, and Mt. Gox is considered too thinly traded for business transactions.
I don't see how the existence of that site, and trading over IRC channels, changes the fact that the bitcoin economy has become centered on Mt. Gox, and in the future, maybe, on several other exchanges. I never said that Mt. Gox is ALL there was.
BTW, I see the reasons that a proper exchange is more popular - there are 3 or 4 transactions an hour on bitmarket.eu, the escrow provisions are weak (as you said, it's for the bitcoin side only), the Paypal fees will be high on the small amounts traded, and trading higher amounts will get negative attention from Paypal - they will not let people trade virtual currency over their systems (and they consider bitcoin to be in that category).
Currently I like Bitmarket.eu most: This site acts as an escrow just for the Bitcoin side of the trade - the "real" money is directly transfered between buyer and seller (via bank transfer or Paypal). That's somewhat more distributed than other sites that act as "full gateways" between buyers and sellers.