I have been using mysupermarket.co.uk for a while to do my weekly grocery shop. I tend to choose one supermarket from the outset, either Asda or Tesco. I have never done an online shop using more than one supermarket as I think it would be too much hassle, time consuming, and of course the fact that I would be paying two lots of delivery charges. I use my supermarket.co.uk rather than the supermarket's own website because I prefer the website layout on mysupermarket.co.uk and find it easier to navigate, along with the fact that I can earn iPoints for shopping through the mysupermarket webiste. You save on petrol by shopping online but the delivery charges range from £3.50 to £5.50 depending on the supermarket and whether you have your shop delivered during the week or at the weekend.
I shop online for convenience, and I do find that I do a much better, healthier and cost effective shop.
Anyone doing their food shopping online for the first time, whether through the mysupermarket website or their supermarket's own website, may find it daunting and time consuming, searching for all the products you usually buy, but once you have done it a few times and set up your favourites, it becomes a breeze.
I have in the past gone to different supermarkets to get different bargains, and found it a lot of hassle. It didn't save me money either as I'd usually buy more than I'd intended in each supermarket.