have to say we found asda great when we used to use them for home delivery. it's our local supermarket and we were familiar with their products so that's why we used them. there were a few substitutions, most of which i was happy with. i rang once when they delivered me something i'd not asked for (or been charged for) and they told me to either use it if i wanted to, or bin it. we used them for about 6 months with christmas slap in the middle, and from the beginning of december we got a 6 pack of mince pies every week for free.
my son uses tesco home delivery and from what he's said, their service is much the same as asda, mostly good but occasional mistakes. either you're prepared to accept that there will be an occasional blip but still worth it for the convenience, or you're not. your choice.