Most times it does not have impact on image. But when there are intense light sources (like sun, bright lights) in the environment, it helps reduce glare and lens flare from affecting the image (http://www.cambridgeincolour.com/tutorials/lens-flare.htm). That said, if you watch out for glare/lens flare, you could also block it with your hands or other objects, or recompose to remove it. A secondary function is for protection.
i just bought the hood for my 15-85 a week ago,and it's really nice for night shot like wedding event.it helps to block away many spotlight.if you intend to buy the OEM,then i suggest you to buy the china made hood.it's relly cheap/and i see no difference except the "Canon" brand only.
i bought with this guy http://www.clubsnap.com/forums/showthread.php?t=658664
u may try to have a look!!