Stacking shelves is a fairly easy and stress-free job. My coworkers are nice and helpful, although management is a different story. My manager has a tendency to exhibit rude and unforgiving behaviour towards their underlings. Pay is just above minimum wage, which, for the job, is pretty fair.
My typical work day is long and monotonous, although I go home knowing that I have been productive. There are long (15 and 30 minute) breaks, although they are quite limited for the amount of time spent working. The hardest part of the job is the treatment by management (at least for me), and the most enjoyable is the job itself.
As far as a high school job or basic part-time job goes, being a Longlife Assistant at Countdown is pretty decent (if you have the resilience to deal with management).
Long breaks, somewhat casual
Long hours, understaffing, rude management