I keep hearing more and more about them, and I'm about to bite the bullet and go with them.
They are a lot more pricey than PayPal. They charge about $50 to get set up, charge 45c instead of 30c per transaction, and discount the transaction 5.5% instead of 2.9% - and then turn around and keep a reserve as well.
But a sale on those terms is a *lot* more profitable for me than a no-sale on PayPal terms. I have never had any complaints about PayPal, and think the anti-PayPal crowd mostly (not always) consists of scammers that got caught, but I've never gotten very far arguing with the weather.
A guy I know says that when he started offering 2checkout on his site, his business doubled immediately on the same traffic. Half of his customers use PayPal, the other half use 2checkout. So I won't be dropping PayPal - I will offer *both*.