There's nothing I dislike more than arguing in a vaccuum, so it's always pleasant to run across documents like this one from the World Bank, entitled Education Vouchers in Practice and Principle : a World Survey. Granted, it dates from 1996, so I'm sure that there've been quite a few changes across the globe that won't be reflected in this document, but it is at least better than nothing, and it does give more than a single-country picture.
A tax funded voucher system exists when governments make payments to families that enable their children to enter public or private schools of their choice. The payments can be made directly to parents or indirectly to the selected schools. The usually stated purpose is to increase parental choice, to promote school competition and to allow low income families access to private schools. Some opponents object that vouchers will destroy the public system, aggravate the poverty problem and encourage segregation. Others fear that voucher-receiving independent schools will be regulated out of recognition. Evidence collected in this paper of 20 countries, provinces or states, so far offers no clear support for the negative predictions. The typical voucher system, which is one in which governments subsidize " schools of choice " in strict proportion to enrollment, appears to be contributing to the growth in quantity and quality of schooling. This type of voucher has been adopted by developing as well as developed countries and notable examples of it are found in Chile, Colombia, Bangladesh, Guatemala, Belize and Lesotho. Elsewhere the most striking cases occur in the U.S. (Milwaukee), Poland, the U.K. and Sweden.The UK did in effect have a voucher system with the "assisted places" scheme put in place by the Conservatives to enable children from low-income families to attend the many great public* schools, but the Blair government abolished it in the name of "equality über alles" once it came into power in 1997; quite how depriving children of the possibility of attending schools that would otherwise have been beyond their parents' means would serve to boost opportunity was never explained, however.
PS: The Adam Smith Institute also has a (more up to date) paper on the issue, titled Learning from Europe (pdf).
PPS: Also see this 2003 digest from the NCPA on the Swedish experience with school choice.
*I.e, those labelled "private" everywhere else in the world.