The Economist has a great story this week about education vouchers and why they work, including some useful data points. If you're interested in vouchers, I recommend reading the whole article, as it includes counterpoints to many of the common criticisms of the scheme.
Here's a quote about adoption of a voucher system in Sweden:
The result has been burgeoning variety and a breakneck expansion of the private sector. At the time of the reforms only around 1% of Swedish students were educated privately; now 10% are, and growth in private schooling continues unabated.
Here's another quote that points out the positive effect of competition.
More evidence that choice can raise standards for all comes from Caroline Hoxby, an economist at Harvard University, who has shown that when American public schools must compete for their students with schools that accept vouchers, their performance improves. Swedish researchers say the same. It seems that those who work in state schools are just like everybody else: they do better when confronted by a bit of competition.
One thing to note is that many education voucher systems pay less than the cost of educating the same student at a public school. It's therefore remarkable that voucher systems flourish even when they're disadvantaged from the start.
Update: here are some more links related to the Swedish education voucher system: