I work for Tesco as team leader for .com. Its quite a good service, I think it depends on where you get your delivery from as to what kind of service you will get unfortunately. We have had 0% complains this week so far, considering the target is 3.5% and most stores fail this every week our customers are genuinely happy apart from the odd few who will moan about anything!
The drivers are not supposeded to accept tips but when I was out over xmas last year driving it was a welcome bonus 
If any of you have any problems with Tesco.com grocery orders that you are sturggling to get sorted then give me a shout and I will try and help you out if possible.
Cheers,
Richard
I have often wondered about the Tip bit, I have recently been asking others if they tip.
We sometimes use Tesco deliveries but have noted that they do not come from the shop that we use, Stockton, they come from Hartlepool and seem to struggle with our pet food order, that we can buy off the shelf if we go to our store.
That said we do not use it enough, just been discussing this with swmbo, but you do not get the deals that you can get walking around the shop, though I accept that we spend more in the store if we go shopping.