Here is a link to an article about voucher schools in Milwaukee:
JS Online: Lessons from the voucher schools
It seems to me that there is a lot of potential, and they are mostly getting good results, but they lack one thing - oversight.
Parents may or may not take their kids out of crappy schools. However, the school board should be active in investigating each voucher school, and taking away its voucher status if it sucks.
Otherwise, it seems ok to me.
The downside will be an increase in religious education at schools - but - if that is what parents want, it will be up to them.