I used Costco a number of years ago for my kitchen counters. They also subcontract out, just like Lowe's and Home Depot. But their price was lower.
The local company they used to subcontract to in my area has since gone out of business. I didn't think their (the sub's) customer service was much good, and they made several errors (seaming, sink cut out) too. And Costco wouldn't provide any go-between either -- I had to deal directly with the sub.
One detail I think they really screwed up was that the granite folks didn't finish the underside of my raised bar. It looks fine on top, but when I'm sitting at my sofa, I can see the rough surface underneath that projects outward into the room.
This summer I found some granite fabricator on craigslist to do a wet bar and bathroom sink. Visited his worksite to make sure he was legit first. Went to the granite warehouse to pick my slab. End result - still had problems with this guy too. The sink cutout was only polished on the top edge, not the bottom, so the edge is jaggedy looking when seen against the sink interior. He chipped a corner off when installing the top and after mucking around with mixing up numerous hues of epoxy to fill in the defect (and failing), he finally sanded it down (and cut into the drywall). I am rather picky though, and expect perfection, so maybe this wouldn't bug most people.
When you ask around at different places, nail them down on the details. Some places charge more for different edges, some charge for sink cutouts, etc.
I don't know if paying more gets you a better result since I've always gone the budget route, but I know that I've not been 100% happy with the results I've gotten.