I just cashed in a bunch of Mypoints points for Vacation Miles, then converted the vacation miles to the $50 Marriott certificates. I haven't it yet but I'll post my experiences.

When I called them they said that they need 21 days advance notice unless you already have the certificates (mine are in the mail as I speak). They also said that they will get the lowest rates generally available including promotionsl, corporate, etc. There are some rates that require instant purchase and are not refundable; these apparently aren't available.

$50 seems like a pretty good deal if I can get good basic rates. I'll let you know.

Oh, one other thing. I've also been stacking up gift certificates to Marriott. There waere a bunch of promotions at giftcertificates.com this Christmas season that generally allowed you to purchase a $25 certificate for $15. I figure I now have $300 in Marriott certificates that only cost me $180. (I think I get another $25 cerficate in January for free, too).