I do my grocery shopping online every week. I've used Sainsburystoyou.co.uk, tesco.com (most recently) ocado.com (waitrose).
They're all good. Sites are pretty easy to navigate. All collate lists of your usual purchases, making it quicker to pick your every day stuff (milk, bread etc).
There will be discrepancies. A missed item, an item you didn't order at all (freebie!) a crushed item (Ii've received shopping packed with the bread at the bottom of the bag, potatoes at the top), but most will refund you quite happily.
But mistakes are only occassional, and any inconvenience far outways the misery of struggling round the supermarket, queuing, getting it home etc.
I wouldn't be without it!