My own rule is to transfer out when I reach $500. I've never had a problem, but then again they are NOT a bank and not insured by FDIC. There are a number of horror stories out there, but I'd guess the majority were stupid mistakes on the part of the user in giving out login info through scams, viruses, or stupidity.
Use strong passwords and change them occassionally. Also, beings so huge, they are a target for scammers and theives. Don't even click an email from them or supposedly from them. Always enter the URL in your browser address yourself, or at least verify the URL is in fact theirs and a secure connection.