Their presence in lots of stores is increasing all the time and I really don't like it. If I wanted to work at Tesco / Boots / B&Q etc, I'd fill in an application form like everyone else! I am there as a customer and part of that is not doing the firm's work for free. It will be an excuse to cut tons of jobs any day now, if it hasn't been already.
Plus it doesn't actually seem to save any time, given that half the stuff doesn't scan easily and you have to wait for someone to come and correct the machine.
Plus the repeated recorded instructions like 'unexpected item in bagging area' drive me up the wall.
If I'm invited to use them, I refuse - though so far I have suppressed my desire to rant about it when someone says 'would you like to use the self-service till?' That's why you're getting it here.