A lot of people want to take a practice SAT before they take the real one so that they can be more prepared. Whether that will work for you largely depends on who you are and whether you need the test preparation.
Some people learn by doing, in which case the practice test might help you. Others learn by reading, in which case the books for test preparation would likely be more helpful than the practice tests. Some practice tests are also very different than others, and whether they help you often depends on that, as well.
SAT practice tests are reliable in some instances and not in others, too, since it depends on the company or person that is offering the test. They do not use actual questions that you will find on the SAT, so you can't cheat by taking the practice test. However, the questions are similar to what you would find on the real test. This gives you a feel for the kinds of questions that will be asked and the way that they will be phrased, which can help you in the long run, especially with nervousness and anxiety.
