Similar stories to everyone else here.
We started to shop with Sainsbury's online, mainly because our local Sainsbury's is in the middle of the town centre and quite small, while the one they deliver from is fifteen miles away and really big, so we were getting a much better selection of items that we wouldn't be able to get otherwise.
It started off really well, but they then started to substitute more and more of our items each time we ordered (and got worse about the replacements they picked - we order a 24 can multipack of Coke, they replace it with a 2 litre bottle of Diet Coke?). Plus they started to give us items that it was almost like they were the last ones left on the shelf (all the other 'in store' customers had rejected them) e.g. items that were very close to their sell-by dates, 'fresh' produce that wasn't of the highest quality etc.
So we went back just to shopping in store again. I wouldn't rule out using it if we were so busy one weekend we couldn't do a shop, or say schedule a delivery for the day we come back off holiday, but I don't think I'll ever do it on a regular basis again.