I wish I could try Ocado, but I can't! Mind you, even if I could, I don't think I would want to use them for basics, because they are not cheap. Waitrose is fun for all the things which the other ones don't sell, and I always spend a small fortune when I go there. So I am quite glad there isn't one too near me...
Anyway, I have used Tesco and Sainsbury online, although neither recently. I thought Tesco was very good, but it depends at least partly on who is at the other end, it would seem. I have had some very strange substitutions - one of the best was cod in parsley sauce which turned up in my shopping (I am vegetarian) instead of parsley!