We (family of 4) shop at Costco every 2 weeks; get all of our staples, fruit/veggies (we eat a lot of that and they have a very good selection of organics), cheese (good selection, much cheaper than other stores); great deals on wine too, and sometimes beer when they carry our craft beer of choice (our local Costco has a lot of nice craft beer), dog food, etc. Some favorites we don't find elsewhere at that quality/price are the Kirkland organic tortilla chips, toasted seaweed (kids' favorite), organic flour tortillas (freeze them and they last), roasted almonds, dried fruit (figs, apricots), reduced sugar fruit spread, etc. For a big wholesaler we find they have a very good selection of healthier food (as healthy as you can get in the US, but that's another story), at least in our city. I find the layout to be pretty consistent and we know where to go to get our regular purchases. Often along the way we'll see something we haven't tried before and get that, though you do sometimes find that they stop carrying it, or it was seasonal, but I can live with that.
What we don't buy there is meat, fish, milk, eggs, all of which we buy at local grocer (sourced locally), plus we're not big meat eaters and don't want to fill up a freezer with it. Besides that, we get some specialty stuff at Trader Joe's (a nice place to shop and the best checkout staff ever -- I think they attract the right kind of employees, a bit like REI). And that's about it. None of those companies are perfect but I feel like they pay and treat their employees well. We shun Safeway/Albertsons/Kroger, and don't get me started on Walmart. I truly hate Walmart for having destroyed so many US towns--will never give them my money.