We did use one at Tesco yesterday morning for a small number of items, but we needed the intevention of the assistant several times to complete the purchase.
First thing to come up was "unrecognised article in the bagging area".......That was the reuseable carrier I'd placed there. The assistant advised us to scan the first purchase and place it in the bag before putting the bag and contents on the bagging area.
At the end of the transaction, I apparently removed the bag too early and the assistant had to reset the machine. Then it wouldn't scan the clubcard or allow entry of the number of own bags used and required the assistant again.
All in all, I wished we'd used a human from start to finish.