I praised HMRC regarding PAYE recently. It's not possible to do so on tax credits. As was noted by the Guardian today:

Gordon Brown's controversial tax credit scheme has been defrauded by claimants of more than £3bn and has overpaid people by £6.6bn since it was introduced in 2003, according to estimates released by the National Audit Office yesterday.

As a result Sir John Bourn, the comptroller and auditor general, has qualified his opinion on Customs and Excise's accounts for an "unacceptably high level of fraud and error".

That's serious. But it's so serious that one has to ask if the error is systemic. I approve of tax credits. But with this failure rate the question has to be asked if the delivery mechanism is right.