The online-only bank Simple (formerly BankSimple) generated a lot of hype when it announced itself in the spring of 2010, and users started signing up for the waiting list in droves. Two weeks ago, Simple started its first mass release, unsure how many of the early interest would carry over into customers — but the company says it's signed up thousands. "The rate at which folks from our invitation list are becoming customers has exceeded all of our expectations," cofounder Josh Reich wrote on the Simple blog.

Simple, founded in New York by Reich and former Twitter API lead Alex Payne, aims to disrupt the retail banking experience with an online front, user-friendly mobile apps and a universal ATM card. Although the no-ATM-fee gimmick is already available from investment-focused banks like Charles Schwab, Simple was exciting because it promises to apply the lessons of good, modern design to improving the lives of bank customers.

Simple is partnering with federally-insured banks on the backend. But it pays to be careful when you're handling other people's money, so Simple has proceeded slowly and deliberately. In May of 2011, Simple released its first cards to friends and investors. In December, it started sending out "a few dozen invites at a time." The company rolled out its iPhone app just a few months ago in May. This is the first time it's opened up on a mass scale to the general public (although not all at the same time; the invites are still coming out in waves, and non-iPhone users will have to wait longer).

The good news came with an odd sidenote: Payne is stepping back from his role as CTO and will become an advisor. "Alex is an exceptional person and I am thankful for the important contribution he has made to Simple," Reich wrote. Simple has had some growing pains, as its lead engineer Dave Fayram abruptly parted ways with the company last year a few months before the company moved from Brooklyn to Portland.

It will take Simple some time to work its way through its waiting list of 125,000. But as cards arrive in the mail, it seems the company that was once merely an ambitious idea has finally materialized.