I bought a zenfone 5 about a year ago, and though it has a dual core processor, it runs very smoothly as compared to many quad core mobiles. Its user interface is very pretty and has a sleek design. In the past one year there was not a single time that I remember that the phone would have hanged.
The only disabilities are the low battery backup, and low sound quality of the speaker. It is currently having android version 5.0 lollipop, and will upgrade to marshmallow within a year.