We used points to cruise in July 2008. Booked a 4 nighter in an "inside deluxe" Cat 10 bcuz we were able to snag one of the "secret porthole" cabins. Then we booked the 3 nighter following it on cash (in the same cabin). We booked it 16 months out in order to snag that room. Since
DCL uses tiered pricing, by the time we sailed, the cash price for the 4 nighter was up to approx. $4,000. So by using points did keep $4,000 in our bank account. However, it cost us 392 points. We only have 250 points. So those 4 quick nights used over 1 1/2 years worth of our points!! We ended up HATING being out of points and took 2 trips to WDW using the DVC cash discount for our accomodations. NOT the wisest move we've ever made. If we hadn't blown the points on the cruise we would've had the points to book our rooms.
Since the 2008
point charts, they have restructured and re-allocated the cruise charts. That same 4 n cruise in July has gone up by 81 points for 2010 to I think 473 points!!

And this is for an INSIDE cabin remember for only 3 people. Personally, I am no longer a fan of cruising on points. For us, that would only be 27 points shy of 2 whole years worth of points!!

For only 4 nights...not worth it to me. We could have 4 whole VACATIONS for that!! NOW, they HAVE made some "seasons" more reasonable to cruise if you can go in an off peak time. But if you can only go in the summer, like us, not such a good deal.
OK, that said, if you have a bunch of developers points to burn then I'd say go for it. No harm done. But if it's coming out of your OWN points you'd better consider the value carefully.
We love DCL tho so we re-booked onboard last summer with a dummy res. for the cash discount and OBC. The re-booking discount is 10%. That can amount to A LOT of money on a longer cruise in a better cabin (verandah). A 7 nighter in the summer with a balcony could easily run $8K so that would be $800 off which would help a great bit. Plus I think the OBC was an additional $200. I'd still rather just plan to spend the cash. That way we can use our DVC points to add some land days to the trip before or after.