From a UI point of view, iOS is very well designed and has had years of polish from the overly flat iOS 7 to find the balance between minimalism and skeuomorphism.
The UX, however, has only gotten worse since Jobs' passing due to the prioritization of surface-level simplicity over genuinel simplification of complex computing tasks.
Just scroll down to the "Links" part to read Richard Mori's articles, if you'd rather not read additional fluff.