Of the two sizes I've shopped, the Costco Interstates were comparable to the "better," if not "best" level JCI FLA* batteries found elsewhere. They don't give all the specs for every size, but the ones they do provide correspond to the higher tiers.
Price wise, they come out on top. Only Sam's Club, Walmart and a code-discounted AAP/Carquest come close. For the one I'm shopping now, AAP actually beats them by about six bucks, but that's with the whopper 40off100 coupon.
42 month replacement warranty is about as good as can be found now. Typically it's 36 months for upper tier batteries. Some expensive AGMs carry 48 month warranties. NAPA favors long prorates over replacement, which is only 24 months even on their better batteries.
I think the one gotcha about the Costco batteries is that they require a current membership to be valid, so if you stop being a member during the term, so does the warranty.
* Costco doesn't sell the AGMs. Google reveals a topic out there on a Saab forum where Interstate support provided incorrect information about a particular size being an MT5 AGM battery, which it isn't. Aside from the branding, I don't think there's any connection to Interstate at all. All matters with these batteries are handled only by Costco.