I've used campusfood.com and foodonearth.com; Typically the orders get "faxed" in by the online service to the restaurants. I have had mistakes before - when ordering more than one of an entree/app/side, it's better to "add it twice" rather than put a number two in front of the box (for quantity). The restaurants typically only see the name of the entree and miss the number next to it indicating you want more than one; So you end up paying for two (or more) of the item and only getting one - that's just carelessness on the part of the restaurant.
Pro's:
1) You can see an online running total including delivery and tax
2) Typically faster delivery (my experience)
3) Rare rare promos where you get credit or discounts from ordering online
4) You're charged exactly what's on the menu
Con's:
1) Missed items depending on how you order
2) They typically ignore the special comments you write (ie: no _______, less _______, please include cuttlery) in the comments box
3) You miss out on specials (ie: free fried rice w/ $25 order)
4) You might miss out on credit card bonuses (for dining out at restaurants) if the restaurant isn't the one processing the order
5) Depending on the service the price might be more expensive/delivery higher than the usual fee