we have tried them all and Ocado is the only one we will use, they do not pick from a supermarket shelf, delivery cost is excellent, we love the different coloured bags.
Yet oddly, when I went round to to help J's aging aunt set up her online delivery with avocado, it was a PITA from start to finish. Managed to set it all up, but couldn't find half the stuff she wanted without doing some very obscure searches, the the delivery was late by a couple of hours [van broke down apparently], which wouldn't have been a problem if they had bothered to let her know with a simple phonecall, and when it did finally arrive, items missing. Now I am happy to accept that this was probably unusual for them, they do seem to have a reasonable reputation, but for a woman in her late 70's it was all a bit too much hassle.
Ref the Christmas delivery thing..... with Tesco [and I guess the others too] you can keep adding to your order until midnight the day before the delivery, so it is worth booking it now, perhaps just with a 'basics' shop then adding stuff in over the next few weeks. Just remember that some of the offers will change in that time so check dates on any special offers before adding to basket.
Also, regarding pickers, as I mentioned earlier, my daughter is doing that job for sainsburys p/t to supplement her student loan and they are monitored for complaints/issues about sell by dates, freshness, correct items, and so on, so most will be doing their best to make sure they don't rack up lots of moans, it isn't good for their job prospects