They aren't too bad. However, they are notorious at giving online shoppers the worst sell by dates.
I have to say that has not been our experience. Usually Tesco and Asda "personal" shoppers whip round the store before it opens and thus one gets the pick of the goods ~ 'specially the veg, bread etc where freshness is important. We normally use one or the other (one likes to be even handed with the tradesmen) once a fortnight for all heavy shopping but buy from local farms for veg, eggs etc. Milk is delivered (don't agree with supermarket milk policies) because we are lucky enough to have a local milkman in a Nissan pick-up who collects from a local dairy at 5am and delivers round the village by 8am.
One big advantage to on-line shopping is that it has reduced our grocery bill condierably. No impulse buying you see. There has also been a noticable drop in the amount of waste we put in the bin each week. On-line grocery shopping forces you to plan meals and shop accordingly. Of course we have no truck with ready meals of any form, unless you count the fish fingers and sausages which the boys would eat at every meal if we let them.