Absolutely. To nearly everyone, the user interface is the program. This sounds like a truism but really is the opposite. It has important practical consequences.
I would add to that the reason I’m wary of shipping an unpolished product as early as possible with the idea of refining it later: you only get one chance to make a first impression.
To use an overused example, the iPhone was unfinished in terms of features but what was there was incredibly polished.