I've tried Ocado (waitrose) and Tesco (I think thats who it was ). Ocado were very good, but then you're paying a premium for waitrose food and there tend not to be many offers unlike other supermarkets (none of morrisons 150+ bogof offers).
What I really object to (and the Wife suffers me going on and on about whenever someone mentions online groceries) is that they charge for delivery. Why should I pay for an inferior service compared to choosing my own, they should be encouraging me to buy online as its far cheaper to operate an online service plus delivery vehicles than a large store. Then they wouldn't have to build stores as big and have as large an area for parking etc.etc. I think Ocado have got it right (stuff comes from a warehouse rather than a store and is selected/packed by properly trained/supervised staff), just a shame they're expensive and still charge for delivery.
I don't think Iceland ever charged for delivery but I've never used them (my freezer's not got enough space for most of the stuff from there anyway ).