Self Checkouts are usually a breeze to use. If I've only got a few items I prefer them to queuing for a manned checkout.
Main issues with Self Checkout are :-
Age restriced products
unregonised barcodes (though that applies to manned checkouts)
The weight for the product hasn't been set correctly (hence unexcpected item in bagging area)
I do know a little of how they work.
Don't forget the super slow customer who doesn't know what to do.
I thought that bit was self evident. One occasion I went into a store (it must have been one of the first day they had them) Operator comes to see if I need a hand, then goes away when she realises I know what I'm doing.