I much prefer only paying when I make money. I don't NEED a merchant account. I just need to be able to collect money from my customers in an automated way. Stripe makes that relatively easy.
Their API has a quick feedback loop, where you can execute a transaction from the command-line very quickly (for Ruby anyway...I imagine it would be the same for other languages).
Their support is also awesome - could be because they only had beta users. But either way, awesome service so far and much better to deal with than a 'traditional' payment solution for those webappers out there (in my experience).
Not affiliated with the company in anyway, other than a customer.