That was the most valuable takeaway for me with installing and managing retail POS systems. It doesn't matter how much time you put into planning and inventory control, if the staff doesn't implement it correctly the POS will never reach it's true value as a tool. It's very important to not only have effective training, but re-training for the humans using the systems.
Depends on the POS system, but older products like NCR ACS, ISS45, Toshiba SurePOS Ace, etc all make huge assumptions that the cashiers will remember 100+ four codes, and can deal with all the curveballs that both systems throw at the user. There are better POSes out there, where you don't have to spend a few days training each new employee for them to be productive and competent.
Training usually isn't bad, Square's POS offering has few features. Remember that you'll still need a receipt printer ($300) and a cash drawer ($100 to $200). Not a terribly impressive offering IMO