Polls are limited in giving accurate results simply due to the small number of individuals who participate in the poll and the demographic. They are administered to only small portions of the population and cannot give an accurate reading of real opinion. While they do give a general idea on how people will vote it is far from accurate.
No, I do not believe that CNN tracking polls are accurate, in fact I believe most polls are inaccurate. The problem with polls is that they are often taken by the primary audience of that news source. A Fox News poll would show republicans in favor where a CNN poll may show democrats in favor.
No, CNN tracking polls do not seem to be very accurate. Neither do other polls. A lot of stock is put into these polls, but it doesn't seem people are really thinking about where these polls come from. Yes, pollsters claim to be using scientific methods to increase accuracy, but they necessarily depend on humans who are notoriously unpredictable. People are not always selected randomly, sample groups do not always reflect the general public, and people do not always answer honestly.
Most news networks have special interests in mind or only poll a certain group of people. Most polls are biased depending on which station you watch. The only real way to find out who will win the presidency is find out on the day of the election when it really matters.