I moved all my bitcoins over to BitStamp (https://www.bitstamp.net/) earlier today. MtGox has always felt a little unreliable at multiple levels. They sound earnest in their post mortems, but it's hard to tell if they are sufficiently competent with web technology to be doing what they're doing.
Bitstamp is no better. If anything, the (comparatively) low trade volume makes it very dangerous. Yesterday, I put in an instant (market) order at about 5pm. It was fulfilled in about 2 hours, for $50 dollars less than the price was when I put it in. I would expect for market orders to be either fulfilled or canceled immediately, not linger pending for 2 hours.
Market orders are relisted every ~10mins at the market price until fulfilled. If you want to sell at no less than some $X, that's what limit orders are for. You used a market order when what you wanted was a limit order; that's not Bitstamp's fault (unless the fault is in making it clear what the difference is, but IIRC they do explain in the FAQ).
