Lollipop for now seems to be manual upgrade by downloading firmware from Asus Zentalk, although I don't think it is advisable as there seems to be a lot of teething issues with Lollipop on Zenfone in general, which explains why OTA is delayed by ASUS.
I am also using Zenfone 5 now, overall am quite satisfied although having used Motorola & Samsung Android phones previously, I think it is quite clear ASUS lacks a lot of experience working with Android software/firmware for smartphones so we can see a lot of back and forth with patching & bugs with every update released.
Another minor complaint is every update keeps introducing more bloatware. Some cannot be disabled easily without rooting. Worst part is even after rooting, cannot delete the apps to reclaim space, can only freeze. Every OTA update checks for all these default apps so if any missing, the OTA cannot proceed. I am a light user so I don't feel very impacted but your mileage may vary.