UI must be designed from the start. Interface considerations infest your design choices, like it or not. Functionality can rarely be completely divorced from the controls which will trigger and modify it. Output must often lead to feedback for the user. The core modes of the application entirely determine the user’s type of experience. Like it or not, we have to realise that as much as it might offend our software engineering sensibilities, for the user the UI is the software.
…Hopefully it’s clear that interface and interaction design is a fine art, and is incredibly difficult to do well. It requires a huge amount of knowledge of your software’s platform, purpose, target market, and indeed of human psychology both in general and in the specific field of human-computer interaction. Certainly, it’s not for the faint of heart, and nor is it to be rushed or attempted without due experience and consideration.
…usability is a lot of hard work spent on ironing out even the smallest wrinkle. It is not something you “spray on” after the fact. It is either part of the design from the outset, or it isn’t. No amount of paint will disguise hard, unforgiving edges.
Usability has no tolerance for hard edges – the only kind of edge users notice is the bleeding edge, and guess what, they don’t like it.
Both are extremely recommended reading for interface designers.