If you are staying five nights, you will have 10 QS credits and 5 snacks. You can use them all on one day if you choose, but I doubt you would. After every meal, your receipt should show you how many credits you have left. I don't think the same is true of snacks, but it's been awhile since we used the plan. Snacks may also be tallied for you.
Worth is very subjective. Some people see worth in having it all paid for and not having to think about it. However, if you do not use every entitlement on the plan, you will probably end up spending more for the plan and convenience of having it paid for than you would if you pay OOP.
The dessert part of the meal, especially QS desserts, can be a little lacking. Most places have pre-packaged desserts that really aren't worth the money. You may want to do some menu research to really get the best value if you decide to do it.
