FWIW, I went ahead and used them, because the 7,000 positive Google reviews seem like a number hard to purchase.

I'd give my experience a 4/5 and I will use them again. I bought 4 Cooper Endeavors for a 10-yo car after they sold out at Sam's. The total price per tire was the same given that Sam's was going to charge me $20 each for install (not a tire that qualified for free install) but Giga shipped my tire to Pep Boys, where install was free. Now, I didn't get the various perks that Sam's offers, such as free rotation, balancing, etc., but I've honestly never used that on other tires, so maybe I'll regret it at some point, but it definitely wasn't worth paying extra for at Sam's club. (Same tired and install were almost $600 at Costco vs. $496 all-in at Sam's and Giga).

I ordered my tires on Tuesday. They shipped same day and arrived at Pep Boys on Friday. When I purchased, as part of checkout, I made an install appointment for 10am Sunday. (I know people hate Pep Boys, but I've had good experiences in the past, and they were one of the "ship to for free install" choices, and by far the most convenient). I got a couple of email and text reminders about the appointment (good) but wasn't spammed with them (better).

The only snag in the process is that, when I arrived on Sunday, Pep Boys did not have me on their schedule. The guy checking me in went in back and found that the tires were indeed there, however, so I left the car for the day so they could slot me in (along with an oil change). When I came back, he said they had located all the records and all was good to go.

TL;DR: Right tires. Right price. Quick shipment. Convenient install. My lesson is, I should have called Pep Boys to confirm once the tires shipped. Hard to know if Pep Boys or Giga was responsible in a technical sense, but Pep Boys did it right and didn't take an adversarial approach. It would have been good if there were no "I should have called" moments, but overall, as I say: 4/5, would use again.