It's definitely not a black and white, "yes, they're necessary" or "no they're useless."
Depends on you. If you learn better in a classroom setting with a teacher and like or need the structure/discipline of a course, then it's definitely worth it. I personally have a hard time staying motivated on my own and wish I'd signed up for a course, because having homework assignments due on a specific date would keep me on track better. I hate the GMAT, so it's easy to slack.
On the other hand, if you prefer learning by reading on your own, can stay motivated, and have solid quant skills, you can probably do fine prepping by yourself.
There are online courses too, which are in-between self prep and a classroom course.
Either way, start with self prep (I have been using Princeton Review's Cracking the GMAT, and it has the best overview I found of the test, plus the tests are really useful, for when I feel like taking one, haha). Spend some time learning about the test and doing practice questions in the Official Guide (get the 11th edition), then take a practice test and see where you stand.