I currently use Simple for my discretionary spending account. I really love the interface. However, if they are serious about converting people to use it as their main bank then their entire focus should be on allowing multiple people on one account. I can't add my wife to my account means this can't be my primary bank. It's that Simple!

...oh and after that, please create a mobile site that allows me to login from any device. I despise having to use my wife's iPhone to check my balance!