A good quotation from a famous person is a great way to lead into a speech or written piece. It can become an effective visual when presented in a larger font on a page, provide motivation or reinforce a learning objective in a training session, or bolster an argument in an op-ed.
But before you go copying and pasting willy-nilly from quotations.com, consider the
Could you pinpoint the location of the quotation if you had to? In most popular writing and speaking, the exact citation is not needed, but you should be able find it if challenged. Some books and especially websites do not provide complete information--just knowing that Martin Luther King, Jr., said it at some point in his life is not sufficient. Warning: if you insert the phrase in Google, you may find anyone who has used the quotation, and not necessarily the originator. This means that your search may involve several steps to reach the source.
Do you have the wording exactly correct? Often, like the game of "Telephone," a quotation gets garbled over time. Example: I planned to use George Santayana's quotation made famous by Winston Churchill, "Those who forget history are doomed to repeat it." Only problem is that Santayana wrote, "Those who cannot remember the past are condemned to repeat it."
Credibility of the speaker
Will the readers or listeners be inspired or horrified when they learn who originated the phrase? Shakespeare or Mark Twain--usually good. Richard Nixon--not so good. The Bible or Bill Clinton--very dependent on the audience. When in doubt about the reaction, it's probably best to keep on looking for another quotation.
The Quote Verifier by Ralph Keyes (St. Martin's, 2006) tracks down many oft-repeated, seldom-cited quotations. The Cassell Companion to Quotations by Nigel Rees (Cassell, 1997) provides context to make an informed call about whether or not to use many of them. Rees hosts a program on the BBC called "Quote..Unquote" and a website with archives and lots of other bits and pieces.
And you already know this, but....you do NOT want to use someone else's quote without attribution. Never wise, but worse these days when it is so easy to verify online.