Battery on HTC One phones have always been their biggest issue for me. I've had my HTC 10 for 2 years now and it's around 60% of its original capacity which equals around 3-3.5h screen on time on low brightness (which is good since bigger one may strain) as long as I use "battery saving" mode. Without battery saving turned on the phone is snappier, but also battery dies much faster (around 2-2.5h SOT for me) and phone gets hot occasionally, ie. not burning hot, but warmer than standard.
I would assume that a new device should last 5-6h with battery saver and 4-5 without it, so it's not a bad result I think. I'm considering buying a new HTC 10 myself within a year as the battery is the only thing I would replace in it, but it seems to be impossible in my country. Anyway - prices for new HTC 10 are currently pretty low (1/2 - 1/3 of standard flagship devices) and it's still a great device - screen is clear and vivid and it doesn't cause me strain, performance is great, it's pretty tough (it lasted several drops with minor scratches on metal) and has good audio if you like to listen on headphones.