MtGox is having its troubles right now. I've used CampBX and Coinbase reliably without any problems about a year ago. I can't speak to the withdrawal process though, other than to say I wouldn't do it through MtGox anytime soon.

There's enough liquidity in the top exchanges that discrepancies in sell-prices from one exchange to another are indicative of people's trust in being able to withdraw their money from that particular exchange.