I noticed some backlight issues on my new iPhone 6s Plus as well. It's at the very top of the screen and you can see a "dash" effect, where the screen alternates between full brightness and slightly dimmed. It looks like tiny pot lights going across the top of the screen. I've contacted Apple about this. They ran me through a full factory reset to ensure it wasn't software related, which it obviously wasn't, but whatever. Once my phone finishes restoring from iCloud, I'm supposed to click on a link in an e-mail they sent me to process a replacement order, which they will ship via Express. That's great, but it means they put a hold on a credit card for the full amount of the device, to protect them in the event that I don't return the defective unit. I purchased the 128GB iPhone 6s Plus using the Apple Barclay card which I applied for during checkout on 9/12 (pre-order day). I was only given a credit line of $2000, so needless to say I didn't have enough available credit for that kind of hold. So I had to get on the phone with the credit card company and get them to give me an extra $500 in credit which would give me enough for them to place the hold on the card. Other than that, I was really happy with the Chat support I received from Apple. This was the second time in a weekend that I chatted with them about something related to this phone, so I think I'm going to go ahead and get AppleCare+. I just wish they had a 3-year option, instead of just two. I'll update this post when I get my replacement device.
Edit: attached image of the issue
