Until very recently, Credits have not really been supported beyond the level of an experiment. That's starting to change, but a lot of functionality and support is still missing. The phase-out of free promotional credits (for which developers were not paid), which was originally slated to occur on February 15th, is ongoing. A functional reporting and management interface for Credits partners would be very welcome - currently this consists of an incomplete API and two .tsv files (a detailed list and a digest of transactions for the previous day) automatically emailed to publishers nightly. These emails started being sent about six weeks ago. There is currently no mechanism for responding to disputed payments or fraud claims but they say they're working on it.
There are positive aspects to Credits though. We see decent adoption but still process many more transactions and many more dollars (at a much lower fee, it's worth reiterating!) through Paypal. The checkout flow is pretty good, and while some people are less trusting of Facebook, some already regard them as a trustworthy steward of CC info, and the latter group is probably more closely aligned with target audiences than the former.
I'd say that right now (and at least until off-site, FB Connect implementations of Credits has been in use for months), it's a question of whether your project requires the other benefits of building on Facebook - user data, social hooks, etc - to the extent that it's worth swallowing the fee & quirks of the platform. Then again, isn't that true of projects built on any proprietary platform, especially one that's still maturing?
Their developer support is horrible. We receive a daily digest with 2 tab separated reports in a zip file, and that's all. Up until recently we weren't even notified that we are getting paid. Once, we tried to contact them about their billing information, and they gave us the wrong answer (which resulted in us paying a penalty to the tax office). I am tending to think they are also understaffed in that department.
We will succumb to facebook's exclusivity request because our app is tied on the canvas, however we are not feeling we are getting our money's worth out of the 30% cut.
It's like what Paypal would feel like if Paypal wasn't a web 1.0 company. (For example, FB is fast -- Google fast -- and Paypal is slow. I'm inclined to believe PP does the auth in the request/response cycle and FB reports insta-success and deals with problems later.)