I finally have a few minutes, so I will break it down and do some ramblin on gear. IIRC, you have a few primes right? If you have a 50 prime, and you are replacing the 18-55 with the 17-40, then pull the trigger.
The good:
Great lens for landscapes, photos of multiple people, full body shots of people. If you use strobes, the 17-40 pretty much makes the 16-35 useless. Additionally, it has a rear gel holder, so you can drop in the cheap ass 5 dollar rosco ND gels to get that glass look to water on landscapes, and you donāt have to spend 100+ on those ND filters. The lens doesnāt change size when you turn the zoom ring, so no sucking in sand or dust when you are zooming. Plus the FTM, and other useability upgrades that you get with L lenses will make shooting easier. On crop, it is a 27-64, which is very similar in practice to shooting with a 24-70 on FF.
The bad:
F/4, No IS, shorter focal range than EF-S lenses that are similar. Like I said above, if you are using strobes, F/4 is plenty, but even then, you have to go to something like a 16-35 for double the price for one stop of speed because canon doesnāt make any cheap-yet-good ultra-wide primes. As for the IS, again if you are using strobes, you likely donāt need the IS, and if you really need the IS, then you are better off getting an EF-S lens, and if you put this lens on FF and need IS, then you need to learn how to hold a camera. As for the focal range, EF-S gots this beat, but they canāt do FF, and they donāt have the build quality of the 17-40, nor do they resist flare like it does. So it is a trade off.
The ugly:
If you arenāt going to use the 17-40 on FF, then you need to save up and get the 17-55, or seriously look at the Tamron 17-50. You can also find 15-85s on craigslist for less than you are planning on shelling out for the 17-40 because people buy the 7D kit and sell the lens. Just donāt buy the 17-85. Personally, I donāt have much use for the EF-S lenses because as you know, I also shoot film, so the red dot is worth the decreased focal length for me.
I pretty much use this lens all the time. If Iām using strobe, or outdoors, or indoors with an open window, Iām pretty much using this lens combined with a 70-200/4. Indoors, at night, and without strobes Iām using my 35/50 prime combo, and if Iām going into a situation where I need to be light on gear, I use the 28-105/3.5-4.5II.
All this to say, ignore all of what I just posted because if you like the 17-40 and have the money to get it, you should. Buy it, shoot a ton of stuff with it, and if you donāt like it, sell it. You wonāt be out that much money, and you should be making these decisions based on if it feels good in your hands and produces the images that you like. Test charts and lens reviews wonāt tell you if you like something.
Here is the lens hood that you should get. Ten bucks, free shipping.
DealExtreme: $10.34 Lens Hood for Canon (EW-83J)