I'm doing it for a number of reasons...live photos, 3D touch, OIS, and the bigger screen. I was cool with the 6, but am looking forward to the extra real estate. I like that you can have your home screen in landscape with the plus as well.
While I have really liked my iPhone 6, over the past year I couldn't help but wonder if I should have purchased the larger model. So, this time I opted for the 6S+. If its larger size annoys me, next year I'll step back down when the iPhone 7 comes out.
Doing it mostly because I love the big, beautiful display and the significantly better battery is just icing on the cake. Ever since I got used to the size of the 6, the Plus doesn't seem so big and I've been wanting to get one for months but held off till the 6S Plus was released.
I've got a 6s and 6s+ on reserve for the upgrade program that I'll choose between on Friday, but I anticipate going with the +, for the reasons mentioned. I find myself working a lot more on my phone than before, so I think I'll appreciate the additional real estate.