It is a great tool. Be careful of the FB guidelines for hosting giveaways sweepstakes on their site. There are some rules about how you promote - do it wrong and you can lose your page. RC has a good blog post on the DOs and DONTs. Rafflecopter's been great though.
They are still rolling out features - would love to see the different winners displayed when there are multiple prizes. That part can be a little confusing.