Ask HN: Are sweepstakes legal and a good idea?

Hi, to collect e-mails for e-mail marketing, I am planning to offer users the chance to enter a sweepstakes. I'll have an checkbox below the e-mail input element which will allow the user to opt out. The prize is a 1 year fully paid subscription.

I was wondering if this is OK from 1. A legal standpoint 2. And a good idea to collect e-mails?