Honestly it can be a double edged sword. One one hand, you can basically dedicate one cashier to check our four guests at once which is pretty cool. But, they can be a headache when things go wrong. You also can't just slap any old cashier on it, you need to make sure that it's someone familiar with the machines and can do basic problem solving skills. They also take forever to close, adding a good 30 minutes to my closing routine. There also seems to be an issue where if you K4 GSTL -> K8, it will fuck up the self checkouts. We have to call the LOD to bring us the 4 SCO start funds before we start closing procedures, and they need to be closed before you close all unused registers. If you don't, then every time you try and do anything on the SCO it will say "Operation not allowed" and they will all need to be restarted before they can be closed.
But when they're working they're wonderful. Closing and maintenance can be a pain, but I think that it's overall pretty positive. At first I didn't like 'em, but once they because part of your routine they're pretty handy.