I've had the S3 for about 7~8 months and and coming from a smaller phone with 3.5" screen to a 4.8" was pretty damn hard going for the first few months until I got used to it and found a case I could use comfortably - the S4 has a slightly larger screen too which has put me off upgrading to it and most likely won't get an S5 if it's any larger.
I'd love the S4 packed in the HTC One body, the only really disappointing thing with the HTC One is the 4MP camera, still, I can't wait to get my hands on it when it eventually arrives (sold out and expected early April, so they say)! It uses an "Ultra Pixel sensor", meaning it can capture more light than an 8MP camera, making it good at low light shots - however the detail is lower than an 8MP sensor. The S4 has a 13MP camera... I think HTC cocked up there, they should have had an 8MP Ultra pixel sensor rather than the 4MP.
