I hope you realize that you're essentially performing a minimum wage job when you scan out your own items. That's why the stores have them. Those machines have a pretty hefty up front cost, but they pay themselves off rather quickly.
I'm not saying it's wrong, but it's just another example of how the stores have adapted to use technology to benefit themselves, not the customers. You used to be able to find someone to bag your stuff and help you take it to your car. Good luck with that anymore.
I'd like to see stores start offering 5% off _all_ the items that I scan myself. I know that with their razor thin margins they probably couldn't do 5%, but something, anything would be nice. I usually just use the machines because it's faster than a clerk.
Overall, I think it's a good thing. This frees up the grocery clerks to do slightly more meaningful stuff... like supervise 4 machines.
Stores do this to cut overhead, and yes once the scanners have paid for themselves, these same stores often start having increase items on sale, or item price cuts across the board. They could do this more if people would; walk their carts back into the stores or at least to the cart corrale, stop putting items on random shelves when they decide they don't want it after all and or at least just keep it 'til they check out and let the cashier know they changed their mind; stop littering in the parking lot including not leaving trash in the carts, stop writing check or at least have it made out except for the amount when they get to the line.
So it helps both the stores and the customers.