As a graphic designer, I hate clip art. Clip art is a pre-made image used to illustrate any medium. They are usually mass produced by desktop publishing companies for quantity, not, quality and as a result are pretty tacky. The one good thing about clip art is that you can use it for yourself or business without worrying about usage restrictions or copyright issues. Most people don't realize it is stealing artwork by using images found online without giving credit to the artist. So yes clip art is free, but it's never really a good idea. It shouldn't convince someone that they are creative and don't need to pay a professional. I know I'm bias as a designer, but I hate to see successful business owners that want to save money by making their own business cards, brochures, etc with clip art and default text. You can always tell they were made by someone who didn't really know what they were doing and should have just paid money to buy something professionly done. Using clip art in professional media just looks immature and reflects badly upon a company that might actually be very reputable. Use clip art to make your garage sale sign or party flyer for work, but nothing that is really important!
Check out this online article:
Clip Art: Cut it Out!