I wouldnt change the Zen UI. Cause it's optimized quite good, and graphics are butter smooth . I tried different launchers on Zenfone 5, and they werent as smooth as the ZenUI.
As for the case, i bought a Nillkin sparkle , with a smart cover thing. It works ok. saves me the trouble of turning off the screen everytime.
Asus updates frequency is ok i guess. It was updated several time since i bought 3-4 months ago. Also android 5.0 update is about to be released. (for Zenfone 5)
Looking at the anandtech preview, CPU performance looks way better than i expected. Mostly the critic is that cpu cores don't turn off completely like Snapdragons ,instead they just throttle the frequency. Does anyone know why?
Weak part seems to be the camera, but for the price , i think its acceptable.