The company’s biggest challenge, banking analysts say, will be to persuade people to give it a try.

“It is extremely difficult to get consumers to change and leave their banks,” said Jacob Jegher, an analyst at Celent, a research and consulting firm. “Plus, although they are not a bank, they still operate like a financial institution, and they will face challenges that big banks have decades of experience with.”

After the financial crisis, smaller community banks and credit unions gained customers eager for alternatives to larger corporate banks. Experts say Simple could attract those customers as well.

Early adopters are warming to the service; during a speech last fall at a conference aimed at technology enthusiasts, designers and creative people, Mr. Reich asked how many in attendance were Simple customers. A majority of the crowd raised hands.

Mr. Reich said Simple was keeping its first group of customers small to allow it to work out any kinks. (Already there have been some flaws, like one that briefly locked several users out of their accounts in November.) At this stage, those who want a Simple account have to request an invitation on its site, though these are handed out fairly liberally to those who meet the minimal qualifications of Simple and its bank partners.

Customers receive a plain white card that can be used like a debit card. The company offers most traditional banking features, like direct deposit and money transfers. But there is plenty it does not offer, like joint or business checking accounts, or paper checkbooks, which may be a deal killer for some.

The start-up does not have physical bank branches or automated teller machines, nor does it plan to build any. As a result, Simple customers cannot make cash deposits and must rely on the Internet and phone for service.