About 9 or so years ago I first tried food shopping on-line with Sainsburys. The experience wasn't great, you had to get a CD sent to you with the products on, then wait for the product update over your 56k modem every time you started up, and then not receive half the stuff you ordered, or find it substituted with Sainsburys own brand stuff.
Skip forward to 2008, technology has moved a long way as has user experience design, and I think this is something Ocado have nailed!
The shop is great, selling Waitrose products as well as branded items which are price matched with Tescos. Every item has a photo and google esq fuzzy search allows those hard of spelling to find even the most obscure sounding herbs and spices.
Once ordered you can pick a delivery time to the hour, update your order if needed, and you even get a calendar attachment on your confirmation email!
The only down sides I have had so far is the £40 minimum to get it delivered, oh and the fact I got locked out of my flat when the guy came to the door last... can't really blame them for that though!