There are always tradeoffs with design, and we make our tradeoffs consciously at Exec. Our priority is iterating quickly, with a design that is presentable and functional. For us, the 80% design done in a week is better than the 100% design done in a month.
Everyone's tradeoff is different - for apps that differentiate themselves on design (Clear, Paper, Flipboard) then it's worth it to take the extra time to get to the 100%.
As Coryl mentioned, I am definitely not discounting the value of design school, I have a ton of respect for the designers who come out of design school and I'm considering going myself one day.
Design school is expensive, both in time and money. It's 4 years of time and tuition is $$$$. Not everyone can do that. A lot of people just want to learn themselves, and you CAN get good enough to become a professional by self-teaching.
My design level isn't up to the standards of devilshaircut - heck, it's not up to the standards of myself yet - but it is up to the standards of many quality companies who are hiring. It's good enough to start a design career, and it's up to you to keep improving through the years.