Interesting, but not the most important aspect of a good game. I don't play games because they;re challenging, but because they're entertaining. And I think challenge is only a small part of what makes them entertaining ... it just makes the gamer last longer. I think more important aspects of what makes a game entertaining is variety and exploration. In one sense, having generated content does make for a lot of potential variety, but I think adjusting the difficulty to meet the user's ability would only make it more frustrating. I'd rather just have generated content at a fixed level of difficulty. In another sense, though, generated content can't have as much variety as manually generated content because the generator will probably never be as smart as a human until computers are more creative than humans.