I'm in Toronto and my wife picks up ribs when they are on sale (no idea of the $$$) at Costco. I find the backs pretty meaty and can't complain. The spares tend to be a little thin on what I would call the rib slab and pretty thick on the "Other" slab.
Oddly enough, here they seem to slash the membrane side on spares but not backs. The backs pull, from small to large side, easy as can be. The spares are slashed so you have to do a little more work. Sometimes I just give up but I do notice the difference later.
Bang for your buck, I can't fault the backs. They are fat on the big side but, unless you're going for a comp finish, they seem great to me.
I am doing my first amateur comp cook in June so I've been practicing with Costco backs. It's a back rib only comp. So far I can only get 4-5 comp worthy ribs from the middle. When I do the math, I can only submit 6 ribs which is tough from a single slab if you comp cut them. We only have a 9" X 9" box to present, no garnish.
Long story short, Costco ribs have been as good or better than what I get from my butcher and bulk meat supplier a few minutes away.