We use Zombaio and CCbill. Happy with both for the most part. Ccbill is always top-notch with everything, that's why they're more expensive. Zombaio is a bit cheaper, but the support is not quite on par with CCBill's. We have both in our sites' cascades, and routinely switch between the two if sales are slow on the other. Not sure why that is, but it seems when we make the "switch" sales pick up on the other.
We're adding Epoch this month just as a 3rd option. I'll sleep better knowing if one---or even two---of our billers fail for whatever reason, its a simply flick of a switch in NATS and we've got a 3rd one. We had Verotel before as a 3rd, and they were OK, but their support is a bit slow and their site is pretty out-dated.
We also offer Ukash for a 4th option. I dont even know if we've made any money through that yet though, I'll have to check, lol
But you really can't go wrong w/ CCBill and Zombaio is a good 2nd choice.