luvmylabs - It's a good idea, here's why:
Although every place will take $US, you should use $J for off-resort spending. Is a much better deal because when JA businesses convert the price of drinks/food/souvenirs from $J to $US, a lot of them do so to their benefit - on both price and change. It's because every business gets to set their own exchange rate - they don't have to use the official one.
For instance on price. Red Stripe on menu might be listed something like either $J150 or $US2 (although $US2 is $J170 at official rate of about 85/1). Or entree for $J1000 or $US13.50 (although $US13.50 is about $J1150). If you take out your calculator, you will find that business is using $75J/1US - not cambio rate of about 85/1.
Same with the change - you pay for that $US13.50 bill with a $20, you are due $US6.50 change ($J550 at 85/1). But they always give change in $J and using their 75/1 rate will give you just $485.
You lose on both ends - price and change. Only if you are carrying a calculator will you notice it. Over the course of a week or two, spending $US100 or more on food/drink every day - it really does add up.
I've seen where some reviewers complain and say they are cheating them doing this. NO - it's just that the business gets to set the exchange rate. Know it and avoid it by carrying THEIR currency.
Actually on last couple trips, I figured out another way to save on the exchange I had not thought of previously. I had them leave my hotel bill in $J before paying my balance due with my Credit Card. Credit card exchange rate fluctuates daily and is one of the best given.