We used them for our current trip. I used the link from disneynutz. We will definitely use Instacart again.
It is very easy to use. Compared to other services we've used on prior trips, the prices are more in line with supermarket prices. There is also a wide selection to choose from. You can also order alcohol, it's from an ABC shop so prices aren't inflated also. Each place you shop from will provide a shopper. They will update as the shopper is progressing. Out to shop, shopping, shopping complete, out for delivery, and at location for delivery.
The only "problem" was having to be present for alcohol delivery. But, I scheduled the delivery a little bit after I knew we would be arriving. After showing my ID, it went to bell services until our room was ready.