I know several friends who wouldn't use mint.com.
For them, it is a single point of failure. You can explain all the technical details you want but there is no way to get past their fears.
When shit hits the fan, you can expect the US govt. to bail out Bank Of America. SimpleBank? I am not so sure.
"However, BankSimple is not a "bank." We partner with chartered banks who provide FDIC-insured products, leaving us free to concentrate on designing the complete consumer banking experience, via the web and your smartphone."
Businesses contract out services all the time. In this case, BankSimple is using some established company to store your money. They just provide the web interface and someone to yell at when there is a problem.
The very first thing I did when I hit BankSimple.com was CTRL+F "FDIC". One tiny mention in a blog preview 2/3ds of the page down. That doesn't cut it.
It's BankSimple's job to inspire confidence in their offering. Their front page seems to spends more time talking in vague terms about how their service is going to be mobile centric, and how they're going to put me first. What their service is all about, and how it works -- the things I care about -- are buried and quite vague.