It significantly increased the speed of communications between Europe and America and was considered by many observers at the time to be the '8th Wonder of the World'. I discussed it quite a bit in my PhD thesis and have also mentioned it in a few other publications. Here are some extracts that you might find interesting:

1) On the speed of news - from the introduction of my PhD thesis, which explores the coverage of America in the Victorian press.

On Saturday 15 April 1865, Americans awoke to find their country in crisis. The previous evening, Abraham Lincoln had been assassinated in a Washington theatre. Within hours of the President’s death, details of the “dark and bloody tragedy” had begun “trembling over the wires” of the country’s telegraph network. By 10am the following morning, flags in San Francisco were flying at half-mast. By midday, newspapers in the East and mid-West had begun to publish detailed eye-witness accounts of the assassination. The story continued to loom large in the American press throughout the following week; the hunt for John Wilkes Booth, the inauguration of Andrew Johnson, and a range of public and political responses to the “national calamity” all commanded extensive coverage. On the other side of the Atlantic, however, news of the assassination was nowhere to be seen. Entirely unaware of events in Washington, the foreign intelligence columns of the London press dissected the closing chapters of the American Civil War – events which had taken place more than a fortnight earlier. The Glasgow Herald even published an unfortunate ‘Address to President Lincoln’ in which a local anti-slavery association wished him health and success during the next phase of his presidency. A week later, the situation remained unchanged. As grief-stricken crowds poured into Washington to witness the departure of Lincoln’s funeral train, London’s Morning Post summarised the proceedings of an Irish cattle show, the Leeds Mercury weighed the threat of a Russian plague epidemic, and the Daily News reported on the Home Secretary’s visit to Newcastle. It was not until the 27th of April that a Canadian mail steamer finally delivered news of the assassination to Britain. By the time Victorian readers had the opportunity to engage with the story, the President had been dead for almost two weeks.

Lincoln’s assassination forms an appropriate opening to this study, for it marks the end of a tumultuous era in American history and the beginning of a transformative period in transatlantic media relations. Sixteen years later, when President Garfield was shot by a deranged office-seeker, the relationship between America and the British press had changed beyond recognition. This time, news of the attempted assassination reached Britain within hours. As Garfield’s life hung in the balance, hourly updates on his pulse, temperature, and respiration were telegraphed to British newspaper offices via the new Atlantic Cable. These updates were printed alongside the latest accounts of the shooting, descriptions of the assassin, reactions from the American press, responses of world markets, and messages of sympathy from international leaders. A President’s death, whilst generating a predictable surge of interest, was only part of a wider journalistic phenomenon. Each morning, the latest news stories from ‘across the pond’ appeared in newspapers throughout the country. Accounts of a political speech in Washington, a devastating fire in Nevada, a gruesome murder in Chicago, a gunfight in Indiana, and the closing prices at the New York stock exchange were printed by British provincial and metropolitan newspapers hours after being published in America.

2) Contemporary responses to cable - from a forthcoming publication on 19th century periodicals:

The transatlantic press was in no doubt as to the profound significance of the event. Articles forecasting the dawn of a new age in Anglo-American relations were published by newspapers on both sides of the Atlantic. Under an eye-catching headline reading “The World Revolution Begun”, the New York Herald confidently proclaimed that the arrival of the cable marked “the starting point of the civilization of the latter half of the nineteenth century.” “Since the discovery of Columbus”, echoed the London Times:

"nothing has been done in any degree comparable to the vast enlargement which has thus been given to the sphere of human activity…. Distance…. is annihilated. For the purposes of mutual communication and of good understanding the Atlantic is dried up, and we become in reality as well as in wish one country… [The cable] has half undone the Declaration of 1775, and gone far to make us once again, in spite of ourselves, one people. To the ties of a common blood, language, and religion, to the intimate association in business and a complete sympathy on so many subjects, is now added the faculty of instantaneous communication, which must give to all these tendencies to unity an intensity which they never before could possess."

“THERE IS NO MORE SEA!”, summarised the excitable Boston Liberator , “ENGLAND AND AMERICA FACE TO FACE!”

Editors on both sides of the Atlantic were quick to highlight the role that journalism would play in mediating this new age of transatlantic communication. “The first revolution will be made in the press”, argued the New York Herald. “[We] look confidently forward to the time when the Herald will contain, besides its local and city intelligence and advertisements, nothing but a mass of faithful telegraphic reports of the events in the whole world of the previous day.” Similarly, The Times forecasted that “within a very short period we shall be able to present to our readers every morning intelligence of what happened the day before in every quarter of the globe.” Eight years later, when a more reliable cable was established, a Kansas paper reiterated the importance of the periodical press in mediating transatlantic discourse:

The ocean is spanned; storms cannot delay nor calms retard the thoughts which one continent speaks to another; we shall hereafter read in the morning papers at breakfast the news of events transpiring in Europe the day before; the two great Nations speaking the same language have annihilated space and time, and will henceforth talk to each other as friends standing side by side.

I think /u/skedaddle gave an excellent overview, but I'd like to add a little bit from my own thesis (only an MA).

First, a great book about this topic is David Nickles Under the wire: How the telegraph changed diplomacy, (2003). It's what got me to write about the telegraph for my own thesis. http://atlantic-cable.com is a great resource as well, especially when you're looking for prices: http://atlantic-cable.com/CableCos/Services/

I focused on the diplomatic impact of the telegraph, especially in war. In some cases, governments were taken by surprise by the telegraph, as newspapers were reporting news before the 'officials' learned about it. I remember one book making the case that many fortunes were made by individuals with the fastest private telegraph lines, I will try to find the source. From my own paper:

To be caught with a lack of information could create a deeply >embarrassing situation for Diplomats or their Governments back >home. One example, before the start of the Franco-Prussian war, >saw a British Ambassador reprimanded by the Foreign Office for >not sending information via telegram. The London newspapers >reported disturbances across Paris, a prelude to future issues. >The Foreign Minister only learned of these from his morning >paper. Without information from his embassy abroad, the >Foreign Office would have been embarrassed had Parliament or >the Cabinet inquired on the situation. Lucky for them, these >questions never came. The situation itself was uncomfortable >enough for the Foreign Secretary to specifically request more, >and more frequent, telegraphic correspondence from their >Ambassador in Paris.

I also looked at the volume of correspondence, to see if there was more frequent contact, in this case between the London Foreign Office and the British Ambassador in Berlin, and saw a jump from 154 dispatches to 675, when comparing 1848 and 1863/64.