I worked at a grocery store for 3-1/2 years, 17-20. I also worked a couple of other part-time jobs while working at the grocery store AND going to High School...
I'm mixed about the experience. Many times, I hated it more than anything else. But those times were when I was under the management of a complete AH and someone that didn't like me because of the color of my skin, others because they had an ego problem, power trip, etc.
While under the management of great people, I enjoyed working there a great deal. I worked better, I felt good about my job and put 110% into it.
I started out bagging groceries. You know, entry-level work. Minimum wage. Within 2 weeks, I was asking to stay LATE (imagine that today) and stock groceries on the overnight shift. Why? Because I was 17 and was saving up to buy a vehicle. My mother wasn't able tot buy one for me on my 16th birthday, pay for insurance, gas, maintenance, etc. and hand me the keys with a large bow on the hood, a $3500 birthday party and all the other stuff....
Within 6 months, I was working in the Produce department (hated it, according to the produce manager I could never do anything right). Shortly after that, I found myself in the meat market. I loved it. I stayed there until I left. A week or two after I had taken an engineering job after college, I was in one of the stores of that chain. I was at the checkout line and one of the major regional managers from the corporate office saw me. He approached me at the checkout line, it was a busy Friday evening. He was literally begging me to come back to work for them. I mean offering me a job and everything in front of all these people. It was kinda embarrassing....
Working in the grocery store and meat department in general taught me a LOT about groceries, meat, life, etc. I wouldn't trade it for anything. I assure you that I know more about groceries, marketing, merchandising, cutting meat and more than most people could ever imagine.