I recently moved to Vegas and having services like this is pretty cool in my opinion. =)
I think the concept is awesome and the convenience is indisputable! Most items are a little pricey and they have a limited selection in comparison to most grocery stores. But, for staples it's pretty awesome. You can change your order up to the night before, it is always delivered before I get up.
They have a random fruit and veggie box that I kinda like. It helps me get new items into our meals without having to think about it. I get items and figure out how to work them in. As other people said if there is a problem they are quick to fix it and it's pretty easy to do it.
I don't mind paying the extra cost for all the benefits but I generally shop at grocery stores that are more upscale/organic anyways, as I feel that the quality of things like meat are appreciable.