Not my experience (I shop in Sainsbury's mostly). I get the "unexpected item" problem almost every single time. Cashiers in my town are so used to it, they don't even look at the baskets anymore - I actually got confused and packed one item without scanning one day - approved with no questions asked (I scanned it later, got another weight error, approved again). Those are becoming as useful as SSL warnings or UAC - just click ok, because in 99% of cases, it's bogus. That's pretty much all they do - almost all the time they're near checkouts, they're busy accepting errors.

There are also products which do change weight. Electronic protection on clothes or wine bottles weight enough to confuse the scales sometimes. Condoms are packed in a plastic container which is taken off after purchase, but the weight is set for the condom pack only. I also get the error quite often because I'm thirsty/hungry and empty half of the bottle, or half a pack of brioches while shopping. (yes, that's my fault but I don't see any reasonable automated solution for it)

Then again, even with all those issues, I can still self checkout faster when doing small shopping alone (15+ items usually means some repacking so I think it's faster only when I'm there with my gf).