I spent 30 minutes playing with both versions tonight at my local Best Buy. The build quality is TOP NOTCH - I was expecting it to be good based on everything I heard...but I was still thoroughly impressed - something that doesn't usually happen to me.
I can't get over how small they feel in your hand. I was planning to buy the S8 but after seeing them in person, the S8+ didn't (to my admittedly large hands) seem big at all. I'm currently lugging around an iPhone 7 Plus and before that I've carried every Note ever made. By far the S8 and S8+ are the most comfortable phones I've held in my hands. They are incredibly well built and stealthy in their size.