A common, folksy investing maxim is to invest in good companies that you understand.  It's not a bad place to start, but as this Bloomberg article on Apple's valuation underscores, the downside is that such companies may have prices that are assuming perfection.  If that perfection doesn't happen, the stock price may be in trouble.  Even if they hit the targets, the likelihood that the investment will grow at a rate greater than the overall market is often low.

Note that this is not a commentary on Apple specifically, or whether or not I agree with the author.  The point is that investing - particularly in individual companies - cannot be done in a vacuum.  The quality of the company is not necessarily correlated with the quality of the investment.  What really matters is how the market values the company.  There are good companies and mediocre ones that make good investments.  The same is true of bad investments.