Mint got away with this by simply being a front-end, and never touching your money. BankSimple should try a little harder to prove that it's not just a company that can identify why banks suck (not all that hard, really), but a company that can actually be relied upon to safely handle your money.
* I know that BankSimple are putting all your money into an FDIC bank, but as the intermediary and the company you are doing business with, the buck stops with them (pun somewhat intended)