We trade out almost every year - Hawaii last year, Atlantis this year. I also have a co-worker who has traded out for Italy, England, Turkey and is currently somewhere in South America. Whether or not it is a good idea, I think it is an opportunity to go somewhere I would not otherwise go.
The key, I think, is be early and be flexible. For Hawaii, we were a little late and didn't get everything we wanted - we ended up actually paying money for hotels in a couple of our stops! But what we got was great.
Work with your rep - they are good.