Summary: What can you say about a piece of plastic that someone is trying to sell you for $35? It shades the 24-105mm Canon lens from glare and looks cool. I know this for sure: if a vendor isn't including it with that lens and trying to sell it to you separately, they're ripping you off because it's supposed to be included with the lens!
Summary: I bought this lens for my 17-40mm f/4 L USM glass as I use a 1.6x FOVCF body (30D and 40D) and the hoods that come with these lenses are designed for full-frame bodies. On crop sensors, these hoods are WAY too wide and I get flare. This hood performed beautifully on my 17-40 L glass with no vignetting even when wide open and with an L39 sharp cut UV/Haze filter on it!
Summary: No need to buy a second lens hood unless wanted. I purchased a lens hood not knowing that one came with the Cannon L Series Lens. I am happy with the Lens Hood that came with the Cannon L Series Lens 24-105 mm but I did notice some black spots in the corners (in my pictures) while using the hood.
Summary: Perfect for shading off light going in the lens. Perfect for lens protection too. Its a waist of money I got a new one but I wasnt informed properly that this comes with the lens. Now I have an extra of this.
Summary: I bought this along with a new lens (Canon EF 24-105mm f/4 L IS USM Lens), both of which are great pieces of kit. I did this following Amazon's recommendation. What a waist of money, as it already comes with the lens kit sold by Amazon.
Summary: Pros: - Reduces flare and ghosting when shooting in bright light. - Helps protect your lens from dirt and scratches. - Somewhat protects the lens in case of a fall. Better to break the hood than the front element. - Makes you look more like you know what you are doing and helps to keep others from carelessly walking in front of you. Cons: - Will increase lens movement in windy situations, kind of like a sail on a boat.