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Dwolla doesn't charge that way because they're not billing credit cards. You give them your bank details and they do an ACH transfer with them, IIRC.

I've heard both good and bad about them, but they don't have large use yet.

You are right. From my POV their problem is that you cannot use them for international projects (non US customers cannot use ACH; non us businesses cannot get their money out of Dwolla)

That's correct on the ACH bit.

In so far as large use cases, they have some really good traction in the bitcoin market. I believe that they are the go to for transferring USD in and out of MT. Gox.

Not anymore: http://www.coindesk.com/dwolla-bitcoin-companies-virtual-cur...

Bitcoin doesn't (practically) charge that but I have no idea who uses them (except few crypto geeks/criminals/speculators)

It's ACH, which is great for fees but has numerous drawbacks as well.