i don't have a marriott but i have a fixed week timeshare that i use in II to trade for marriotts (usually in shoulder season).
if you don't mind being limited to 7 night vacations, i think it's a great option to look into. you might want to join TUG to learn more about locking off a 2BR and trading both the 1BR and studio components for 2 weeks of vacation elsewhere...
if you are not limited to the school calendar, there are some great trade options available.