If this is an attempt to repeat PayPal in 2.0 fashion, then I don't see them grow too fast and too strong. SSN aside, the sign-up bonus seems to be valid only for U.S. account holders. On top of that, there is a limited number of participating banks.

The absence of account fees is certainly encouraging, but if you can't benefit, what's the use? I don't see them really become popular, but I'd enjoy watching them compete with well established rivals on a turf that has been monopolized long-time ago.