Probably going to hold off for now. I had an S4, and now have a Nexus 5, so can't really justify the upgrade just yet.
Will see that the Note 4 and Nexus 6 bring to the table. Also, with the rumours of a larger screened iPhone, I can't actually rule that out just yet.
I've got the urge to upgrade, but nothing has quite grabbed my attention enough yet, they just don't seem enough to be truly noticeable day to day over the N5. Although if I had to go for any of them, it would probably be the S5. The M8 is fantastic, but I just don't think I could get around the camera; I take a lot of photos and like going back through older ones and I just worry I'd miss the resolution. I may not, but it's enough of a worry to put me off. The S5 has its own flaws though, KNOX is pretty much a dealbreaker, but I guess I'll have to get over it as it's becoming more and more standard now unfortunately.