18% interest? I've never looked into financing so I don't know what the going rates are, but that interest rate seems brutal. That's like charging it on your credit card.
As a whole, "worth it" is a difficult question to answer. Depends a lot on your travel habits and what kind of accommodations you are willing to stay in.
But I'd be really hesitant to buy if I had to finance at 18%.
Edit: Sorry, to be a bit more helpful:
240 points x $180 per point = $43,200
I assume you put 10% down? = $4,320
$600 per month x 120 months = $72,000
Total paid = $76,320 / 240 points = $318 per point.
240 points at VGF in Dream season gets you about 11 days for a standard view studio per year. Assuming a hotel room is $400 per night,
DVC still saves you money even at $318 per point, however you have to consider whether you actually would have stayed at the Grand Floridian Hotel 11 nights per year every year for the next 49 years.
Edit 2: Assuming $400 per night for the hotel room, and also assuming 4% increase per year for both annual dues and hotel room feels, break even point is around year 18 (2033).