Apple is a rare form of underdog brand. They’ve been able to move from successful niche product (Macintosh PCs) to mass market hit (iMacs, iPods, iPhones).
They made that move not in spite of focusing on building a successful niche product, but BECAUSE they built a successful niche product. The focus, razor sharp positioning, and insurgent marketing techniques they honed as an underdog were the lever for their phenomenal success.
In a brilliant post today, Seth Godin today outlines three segments of the "long tail," including mass market hit and successful niche. He discusses choosing one strategy or the other (or the third option: own the long tail) when you launch a new product or service. This post does a great job of illustrating the differences between the various strategies.
"The most common misconception about Long Tail thinking is that if you don't succeed at pocket 1, don't worry, because the tail will take care of your product and you'll just end up in #2. That's not true. #2 isn't a consolation…"
That's why successful underdogs will almost always choose the successful niche strategy.