Matt pithily sums up my feelings:
But in neither case would it address the issue in a comprehensive way. Which, I think, is one of the main attractions of the voucher concept — it lets people get indignant about the sorry state of public education by basically assuming the problem away, thus avoiding the need to deal with the real issues.
There are a lot of very good, very smart people thinking through education policy, childhood poverty, etc. Then there are somewhat more shallow people who want to propose a tough-minded solution to the sorry state of inner city education, and they fasten on vouchers (which no evidence has ever suggested will actually solve the problem) or teacher's unions (ditto). Those policies may have some worth. But they are not Answers, no study has ever suggested otherwise, and forcing us into an endless conversation over them is actually bad, so far as I can tell, for the education debate. They do, however, give a certain class of participants a useful club with which to beat on liberals and accuse them of active opposition to the disadvantaged.