If you can get it for free it may come in handy when they do deals. I know for the holidays, and I think a couple other times of the year, they do further discounts on Gift Cards. I picked up restaurant GC's for 30% off during this last one. AmEx also did offers a few times last year for online orders, some were better than others but I think they are usually a 20% discount (like get $10 off a $50 order, or $5 off $15, stuff like that). They also more recently added more items that get free shipping even without the Plus membership. It'll probably come down to seeing if there's more national brand stuff there that you use vs Costco; I know the big draw to Costco is their own brands.
I'm not a great response because we are usually BJ's people but my Mom had Sam's Club so I'd just use her membership when I had a Sam's only deal - and it usually involved GC's
. Her membership expired this week so we took the opportunity to get a freebie by me signing up this time and she can just use mine for this year. We can easily find $45 worth of stuff we'd buy there or Bj's anyway and I have $20 pending in Dosh for signing up for a new membership. (I took too long and missed the SD offer on top, win some lose some)