The only real advantage they have at the moment is momentum, and lots of VC money allowing them to survive much higher customer acquisition costs than otherwise. Otherwise, they're a specialised variant of online grocery ordering, which at least in the UK is a huge and very competitive market. They even have the advantage of being able to mail orders, because they're not trying to deliver anything frozen, so logistics is essentially just a case of negotiating a decent rate with a courier.

I'd be really surprised not to see quite a few companies pop up in the same sector, there are already a couple I know of, possibly targeting particular niches such as oriental food.