That does sound almost too good to be true; might want to read the fine print. Often they'll say something like 'up to $100 onboard credit', which means that's what you get if you book a suite, but then when you look at it further, it'll be like $25 for an inside cabin, $50 for an oceanview.
But if it's truly $100 onboard credit for a 4-night cruise, that sounds too good to pass up.
p.s. I saw the sale and it says up to $100 if you sail on the Oasis or the Allure. If you sail on the Majesty of the Seas, you get $25 for an inside or oceanview and $50 for a balcony or a suite. And by the way, it's for Saturday bookings only - just goes to show you've always got to read the fine print.
Here are the details; http://www.royalcaribbean.com/conten..._rid=231853062