Yeah I don't know how I'm going to go to another phone.
- I firmly believe Sense is the best UI on a phone today. Better than Touchwiz, better than Sony's better than stock android. It's absurdly smooth, very beautifully designed and has some really cool features. I feel the most comfortable using Sense. Never looking around trying to navigate to something and unable to find it, not bombarded with settings and icons everywhere, and visually appealing. The UI is so good I have no reason to root/flash anything to it.
- Build/design. This one is completely subjective. Some people prefer plastic, others don't care, but I do. I love that the phone is metal, I love that the phone has the curved back that fits snug in the hand, love the design of it. I look at the phone and think 'damn that looks nice'.
- Boomsound completely changes the experience of the device. I can't stand watching stuff on a phone with a single speaker rear-mounted. Every other phone speakers just sound like absolute garbage to me. Went out on the weekend and a friend was trying to show me a video on youtube on his Galaxy S4 where he had to do that laughable cup-your-hand-around-the-back nonsense. I pulled out the One, found the video and we both watched it. Looked and sounded better than the S4 by a mile.
- The camera. I'm Gen Y without kids, so most of my photos are at night at a restaurant/bar/whatever with friends. I've yet to use a phone with better low light performance than the One. Photography isn't super duper important to me, but seeing night shots turning out really good is impressive. Daylight performance could be better, and the phone does tend to blow out whites on auto-mode, but for my uses, I'm loving it.
No doubt, other phones have their own advantages and disadvantages, but for me, the One is an extremely hard device to beat. I've played with the Galaxy S5 and that doesn't interest me too much, especially previously owning the GS4. The Z2 looks nice, but owned the Z1 earlier this year as well and wouldn't go back to Sony's UI, or their build/design.
Of course, we all have different wants/needs/opinions so the things I love about the phone, others may not care at all, and something like water resistance might be super high on their priority list, which in that case, the One would be of no use. Just for me though, I think it's a superb phone and I don't see myself swapping from phone to phone as I did last year. I'm sticking with the One M8 until the M9 comes out next year.