What is it about the 17-40L ?

[Toby]

I see many people around with this lens and it seems to be held in very high regard but I'm trying to work out exactly why.

The zoom range seems very narrow and it's only F/4, not F/2.8. What is it about this lens which makes it stand out? Is the image quality that much better than the competition?

[divine]

It says L on it.

This means one of two things - it will be good quality.
L Fanboys will rave about it beyond comprehension.

I believe it's one of the cheaper L lenses so it tends to be a lens a lot of people get when looking to upgrade their 18-55, especially if they have got it into their heads that anything non L is rubbish.