I'll be sticking with my GS3, at least for a while yet.
For now, I just see no need to change, and just cause it's newer wouldn't necessarily mean better, as I am sure you know

The GS3 still has some good DEVs working on stuff for it, it's a good stable phone IMO, although some may disagree.
Overall though I like it, and I am comfortable with it, and I guess that would be my biggest thing is how comfortable I am with it.
My step dad recently got one of the HTC M8's, and while it does look pretty sweet, I didn't like it too much when I was helping him set it up.
As soon as it came out the box it was having issues with the volume down button getting stuck so when he would try to power it on, it would power up like he was trying to hard reset it lol. I'm not that big on change xD