I was a bit wary of using self scanners, but after being advised to use them, as a shielder, rather than queue at the checkout. I have found them a godsend.
It also means I don't get stressed with DH, who takes forever to unload items on the belt, usually oblivious to the tutting from the queue behind him, as he dithers! 
I am another one, who adds mentally and have a rough idea, but I still can't help checking, even the self scan ones.
Self scanning is not perfect, when it comes to reduced items. Sometimes the change hasn't been updated on the system..
Oh dear, I sound like a cheapskate! It's just that I love to bag a bargain!!!