The stores I visit usually have 10+ self checkouts. It swallows a lot more people so you don't have to queue like in a regular store. It also allows people with fewer items to bypass the people with a full shopping cart because there are 9 other checkouts.

You also save time by putting the scanned items directly into your bag instead of putting them on a band for the cashier to scan, before you can put them into your bag.

So my experience is that it's faster most of the time.