Well it is a neccessity to have this hood on any of the wide angles lenses that you buy from Canon. Totally agree that Canon are seriously annoying their customers by not providing it with a very expensive lense set (in my case the 10-22mm EF-S).
Apart from the justified rant, this hood is robust but large, well designed and gives the lense a great feel of adequacy. It breaks down the flare and glare completely and you really shouldn't use the lense outside or in most cases inside without this hood.
On a marketing note would we really not choose the lense at around £650.00 if Canon decided to add the hood for an extra £30.00! I think not so shame on you Canon for making the shopping experience that little more difficult for your customers.
Just remember that if you have a number of lenses in your bag and like me you have a hood for each of them, this hood doesn't or probably won't sit inside the other hoods as it is quite large in diameter. However due to the fact that it is quite short in length it should fit nicely on it's side somewhere at the top or bottom of your bag/case where space is restricted or in un-used in most cases.