I have done my shopping online for about 2 years

its GREAT!
Yes there have been a couple of things which have been packed wrong or missing but i've also had some freebies! When i've phoned up i've never had any problems and always got a full refund, even for stuff like grapes and bread which is just a bit squashed.
You can chose to not have any subsitutes but i find there usually ok, you can just get them to take them away if your not happy.
I don't think i could be without my online shopping it saves me soooo much time. I use both tesco and asda. I do prefer the tesco website but i perfer the asda stuff so i alternate, lol.
It takes quite a while first time and you'll probably forget some stuff but after a few weeks it will only take you around 10 minutes as it store all your stuff
If you do decide to give it a go don't forget to search for voucher codes on the net i always get free delivery and sometimes £5/£10 off!
HTH and if you do try it you'll ahve to let us know how you get on, Gemma