With a functional app, you want to test before putting out to early adopters. Early adopters are going to be your biggest fans, so you want to get the experience as close to right as you can before launch. This is where usability testing comes in. Running a series of interviews with potential customers who haven't seen the app before is best.

Doom II, the game from 1994 is best in class, in my humble opinion, even when compared against its successors. The reasons I think so include: