I have personally used it but might now so I don't have to brave the traffic/chaos that a suburban Costco can be. That being said I haven't heard great things about IC in my area.
It, admittedly just now, strikes me that taking away the headache of Costco would be pretty welcome. We get mostly the same things and rarely go there without a list/goals for the trip. Additionally, their sales don't change often. Making online ordering much easier than going to the grocery and planning for the week/scoping the sales.
This really turned into a costco related comment.