When Apple CEO Tim Cook took the reins from Steve Jobs in late August, observers from Silicon Valley to Wall Street wondered whether the longtime chief operating officer could truly replace the iconic company's venerable co-founder. Cook, who joined Apple in 1998 and had handled day-to-day operations since 2005, had answered that very question a few years earlier in an interview with Fortune, saying, "No. He's irreplaceable." But before Cook fully took over, he pinch hit during Jobs' medical leaves in 2004, 2009 and 2011 and by the end of each stint, the company's stock price was in better shape than when he first took over. In fact, under Cook's watch, Apple became the most valuable company in the world during 2011. Irreplaceable though Steve Jobs may be, Apple's in very capable hands with Tim Cook behind the wheel.