Apple have established an awesome reputation for quality and innovation. While they have only about 18% of the personal computer market they dominate the MP3 and phone market and the iPad sets the standard in tablets.
It is very difficult to challenge Apple in these markets and as a result of this barrier to entry (brand loyalty) Apple can charge very high prices.
Amazon have a similar position in book and ebook retailing online.
The problem is that monopoly power (which any firm with a large market share gains) works against the consumer. Competition should bring down prices and raise quality and customer satisfaction, but entering the market is often difficult and does not always work as expected.
Amazon is now taking on Apple's iPad with the Kindle Fire. A combined Kindle and tablet, and it is less than half the price of an iPad.
Is is good for consumers? Only a company like Amazon could really mount a serious challenge in such a market. They are providing an alternative at a lower price. But Amazon are also trying to lock you in to their product - Kindle ebooks. Remember Apple have iBooks, and the two systems don't talk to each other.
So we could look at this as welcome competition, or we could see is as two Oligopolists fighting for bigger market shares at the expense of any other competitors. Both Amazon and Apple would settle for being the Coke and Pepsi of consumer electronics.
My Kindle Fire is ordered and expected delivery is October 25th!