I'd been flicking through Paul Graham's Hackers & Painters over the weekend when I came to this paragraph:
Not every kind of hard is good. There is good pain and bad pain. You want the kind of pain you get from going running, not the kind you get from stepping on a nail. A difficult problem could be good for a designer, but a fickle client or unreliable materials would not be.
Strangely enough I was trying to explain the same thing last week.