I have been using Ocado for approx 2 years and have been very impressed with the service.
Its easy to order either via my laptop or mini ipad. It keeps a list of my favourites so its easy to find my regular items. I can add to my order up till late the night before.
I like the fact that I receive a full list of my order listed by use by date so I can keep it near my fridge so I know when to use items. I also get a text message to tell me who will be delivering my shopping.
The delivery men are always cheerful and polite.
Any problems/refunds are dealt with quite quickly
My one negative is that if they substitute an item they charge the higher amount