All in all, I like Ocado. They're reliable, good quality and everything's very convenient. (My wife does the shopping itself, so I can't comment on the user experience on the web front.)
I do have two gripes however, both to do with substitutes:
1. If you send a substitute for an item because the ordered item is not in stock, tell me how much the ordered item would have cost me as well as the cost of the substitute; and
2. If the substitute item is more expensive than the ordered item, swallow the additional cost. After all, it's your fault, not mine.
The first request is a seemingly trivial invoice display issue. The latter is a point of principle, one which would drive loyalty and bring a feel-good factor to customers suffering at the hands of your stock issues.
