Just to add one data point, I did jump at the deal, but I called Samsung to make sure that my S3 which was still in great condition would count. They said yes, of course, and so I ran with it. Then this thread popped up and I got worried, but sure enough they accepted the S3. The (replacement) battery was amazing and didn't run down so that wasn't an issue given that it did take them several weeks to test it. Anyway, I wanted to post since I figured people who didn't have their trade in honored would post, and it isn't clear at all if that is a huge percentage or tiny given that people who had theirs honored are much less likely to post.
The accepted return shows the s3 as "product shipped" on the Samsung website. They said that they don't send an email if the phone is accepted, only if it isn't.
I ordered on the last day of the sale and got the new phone on July 18th and shipped my phone back that same day.
I hope those who aren't having their orders honored will win the dispute.
I like Samsung phones, but do have to admit Apple has much better service even if I personally find their phones overpriced and boring compared to android. That said for $425 the unlocked s8 is a heck of a deal with or without world class customer service.