I guess it depends.
Fresh & Easy stores are entirely self checkout, with fantastic success across the board.
The key difference seems to be that the machines that F&E use don't suck ass. Get a bar code anywhere vaguely near them and poof, *beep* you've got it. It doesn't take some special practiced skill like the old, crappy bar code readers that many stores still employ. Anyone can wave items past the table and checkout at far faster speeds then traditional checkout personal ever could. And it shows: Despite a steady clip of customers, there's practically never any checkout line whatsoever.
It's in sharp contrast to the self-checkout scanners at Home Depot. You spend ages with each and every item, waving it over and over, spinning it round and around, nothing works. Not even the trained helper who comes over can make it work and eventually just types the code in manually.
Investing in quality equipment makes a huge difference. Most of the places that tried and failed with self-checkout tried to do it on the cheap.