I hate to be overly negative against this startup, but I have a couple of issues with this service, that haven't been addressed since Webvan in 2000: 1) The target audience is definitely those people who are too busy to get their own groceries. If they are too busy to get their own groceries, aren't they too busy to cook a meal? I would think that take-out or going out to eat would be their #1 competitor in this area…
Our metrics to date seem to disagree with your speculation about people not having the time to cook. There are also a host of products that Instacart carries for occasions other than dinner, like sundries, snacks and beer. These are our most popular items. Also, I agree that it would be a terrible experience for the customer if items arrived expired or broken - which is why we take such great care to make sure that those things don't happen.
We've also found that a 3-hour delivery is very convenient for most customers, and it's by far the most preferred option. With regards to delivery from grocery stores themselves: please try ordering something from one in the next hour.