Imagine a customer of your company experiencing a billing problem that can only be solved by Google. Their lack of responsiveness could be interpreted as YOU being unresponsive. Until Google offers you dedicated support, I would be wary of selecting them.
Disclaimer: I work for a recurring billing provider today, but I've had a lot of experience dealing with different billing solutions.
Edit: I started an "Ask HN" on this => http://news.ycombinator.com/item?id=1792799
I had a pretty severe Gmail problem once and I remember trying to contact someone or report the issue, a nearly impossible task. All I found were static help documents. Of course, that was ~4 years ago. And their support for Google Checkout may be handled entirely different from that of Gmail, but it's still something to consider.
Then again, I got burned by the Chargify pricing hike recently, so I can't really advocate to go with the smaller "Chargify-ish" companies, either.
Both of these services only offer checkout capabilities. They don't offer product/subscription management or other advanced features like dunning which gateway wrapper services like Chargify, Recurly, Spreedly and ChettarGetter offer.
Source?