It's great, saves loads of arse-ache and the mrs finds it handy being able to see a running total while crusing the aisles.
The only bugger is when they do a random rescan - you have to get everything out of the bags, have them scan it at the checkout, then repack. I had it happen 5 times in 6 visits at our local Sainsburys so I griped at the customer service bods and had them put me in the "not-a-thief" category. Problem solved.