Coming from the HTC One M7, I have to say I'm really loving the S5. The screen is a noticeable upgrade, and I love how it adapts when reading e-books, it makes it much easier on the eyes. More usable screen real estate is great too. The look and feel of it is much nicer in person.
IMO, this phone feels like a more functional tool, it is easier to hold and the power button on the side is logical. Charging it is super quick, as well as transferring files, thanks to USB 3.0. Made short work of transferring all of my photos. The camera is super nice, much better than the M7. Super impressed with it already.
All in all, I am very happy I made the jump. The design isn't as nice as the M8, but man, that phone is slippery as all hell, and if the M7 is anything to go by, using it while lying in bed is going to be a huge feat! The M8 is also much taller and not having physical navigation buttons was a mistake, as it reduces the amount of screen you will see during browsing and using apps. It's nice to have the eye candy though. I've ordered a Spigen Neo Hybrid case in an attempt to beautify the S5 a bit. Really like the design of that case.