I've been using most of the exchanges for the last six months. In order of reliability for me:
Reliable so far:
1) Coinbase
2) Bitstamp.net (Coinbase's exchange)
Agreed on both of these. I'll note I only have used Coinbase to convert to fiat, rather than the other way around. I have heard of some having difficulties in the other direction. I am about to try buying BTC from them, but will only do it if the price goes down in the near future. At least in some significant amount. Regardless of price, I am going to do a couple buy transactions just to verify a credit card and jack up my limits a bit.
As for Bitstamp, I do not use them personally as an exchange, but I consider their ticker a lot more reliable indicator of value than the often fantastic and unbelievable prices the Magic the Gathering exchange gives. How the Magic the Gathering folks are considered the leading exchange is kind of a mystery, because their ticker is ludicrous. It is hard to avoid the conclusion that these guys have no idea what they're doing.