I remember in Brazil voting was (is) mandatory. When you vote, you get a receipt of sorts that verifies you voted. This receipt is like a piece of gold. You can't open a bank account, register a business, apply to post-secondary, and many other things without that receipt.
I'm not sure how I feel about it. Voter apathy is growing in modern democracies, rendering them less effective. For instance, in Canada's last Federal election, 40% of the country didn't vote. That means that the politicians in Parliament are only representing 60% of the total country.
On the other hand... forcing people to vote means they might not make an informed choice. They'll vote just because they have to, instead of feeling that it's their civic duty.
I think I could be in favor of it. By forcing people to vote, it could make more people become politically aware.