There are two major aspects to this question. First, it asks how much the media in general is able to determine what issues we voters think are important. Second, it asks how much the traditional media, in particular, can influence our thinking.
McCombs and Shaw argued that the mass media were able to exert a very strong influence on what we think is important. They argued that the media shape reality rather than reflecting it. For the most part, the world of politics and issues exists for us only because the media discusses it. If it were not for the media, I would not have known about the sub-prime mortgage crisis of 2007-8. Without the media, I would not have known about the civil war in Syria. Without the media, I would not even know that Al-Qaeda exists. Therefore, the media essentially shapes my reality. It tells me things that I would not otherwise know.
Of course, there are many more realities than those that are portrayed in the media. The media cannot report on everything that is happening in the world. Therefore, the media has to choose what it reports. When it chooses what to report, it shapes my reality by determining what things I will know about. In this way, the media really does have a strong influence on what voters think is important.
However, there are some qualifications that must be made here. The media cannot make people care or not care about an issue if they actually perceive that issue outside of media reports. For example, when gas prices go over $4 per gallon, people will care, regardless of what the media says. As another example, if there are a lot of illegal immigrants in a given area, the people of that area are likely to care about the issue even without the media. It is also true that the media has more influence on people who do not pay very much attention to political issues. People who are highly engaged are likely to hear about political issues from other sources and will care about those issues regardless of what the media say.
Today, there are many more sources outside the traditional media than there were when McCombs and Shaw wrote in 1972. There are so many sources of information on the Internet that we no longer have to rely on the traditional mass media. This is reflected in the declining readership of newspapers and the declining importance of network television news.
So, I would say two things in response to this question. First, I would say that the media does influence our thinking, though its influence can vary based on the nature of the issue and the characteristics of the people who consume the media. Second, I would say that the influence of the traditional mass media is declining as more sources of information become available through the Internet.