I'm a member at the executive level which has a higher membership fee but I get 2% back from most of my purchases at the end of the year. I end up getting all of my membership fee back with a little extra, so it pays for itself.
I get my glasses and contacts through them which is a huge savings over other places. My titanium frames alone would have been 3x the price anywhere else. Prescription medication seem to be less but our family doesn't get prescribed medication much so I can't be sure on that.
There are certain nonperishable items that I purchase in bulk. And I sometimes purchase snacks and frozen veggies that we go though quickly.
There isn't a huge savings on electronics. In fact sometimes they cost more at Costco, but their guarantee and customer service makes up for it. I find their electronics people are way more knowledgeable about their products than the specialty store people. If you're lucky, sometimes the company rep is there to answer questions on their products.
The other thing that is a factor for me is that it's a sort of a local company for us, and they carry a lot of items that are produced in Washington and Oregon. In fact their house brand is named after the town that they are based in and I used to live about 15 miles from there when I lived in Washington. I don't know if I'd buy from them so readily if I lived out of the area and everything they carried was from somewhere else.
Everything in moderation, including moderation.
2007 Rodriguez Adventure/B72
2009 Masi Soulville Mixte/B18
1997 Trek 820 Step-thru Xtracycle/B17