It kind of depends on what you want and what you are willing to spend.
We used to use Netgrocer at home for baby formula and I loved it.
If you are going for the fruit snacks, granola bars, pop tarts, microwave popcorn, a box of cereal, a case of pop, some spagetti noodles and a jar of sauce - and are willing to buy milk and a frozen pizza from the Disney gift shop, Netgrocer will save you time (and time is money at Disney), but probably be a wash or a little more expensive than a grocery stop. If you are planning of real meals, Netgrocer doesn't sell enough to make it worth the bother - no meats (other than canned stuff), no fresh produce, etc.
We order all our groceries at home on line and have them delivered. And the point made above is right. Make a list before you go on line. Its easy to click your way into too much - and easy to forget what you want because you aren't looking at it.