I went from an iPhone 5s to edge 7 for a few weeks.
biggest UX change was going from the swipe from edge gesture in apps to tapping the back button on the lower right below the screen to go back in the apps UI. took me a little while. muscle memory is real. Also, the prevalent use of the upper left corner menu button is tough to reach right handed on a 5.5 screen. I still prefer a lower nav bar.
aside from that, there was some time figuring out if i wanted to use widgets a lot on the home screens (I still find them to be only mildly useful). and I probably didnt make full use of the two-apps-on-one-screen feature. i tried it a couple times and it would be useful for some things but i still think a 16:9 5.5 screen is too small to spend much time with that feature.
some nice apps didnt have options to pay to remove ads. and when, for an RSS reader, the ads were occasionally full page interstitial ads, that was an annoying omission.
regarding fluidity and responsiveness of the UI, the device seemed pretty solid and I had no concerns, really. much better than my last jaunt into Android land ~4 years ago.
Ultimately, i went back to my smaller iPhone for ergonomic (and some niche webapp compatibility) reasons but it was fun to explore android hw/sw for a little while.