I don't use Netbillings personally however one of my clients does. And they've always done really well for him. I know when I've needed support from them they're fairly quick to respond (within 24 hours) and offer a lot of features.
As far as rates go I'm not certain, since as I said I don't use them myself except on behalf of this other guy, I've never looked at the costs since it's not my money
