As I own and use all three, I get MANY private messages on various forums asking about my thoughts between the three lenses. So I thought, if for nothing else, this post is just to have somewhere for me to point these people to in order to save me time.
If you have also OWNED and rigorously used ALL THREE lenses for at LEAST A YEAR, then it would be great to have your comments too. As exciting as it may be for someone to say "I've had my 24-70 for two weeks and so far and it's been great", it just doesn't help bring experienced opinions into the thread.
This is my very unscientific opinion of the three lenses. I don't take pictures of focus charts and signs and do technical tests. I turn to LightRules when I need to find good information of that type. No sense duplicating. What I *DO* concentrate on is their usability and performance when shooting... I like to get out there and use my gear to the fullest, and I expect it to keep up. Some characteristics of the lenses help me, some do not.
Also, bear in mind that I'm mainly speaking for 1.6 crop shooters. This should be obvious as the 17-55 doesn't fit on anything else.
Anyhow, these are my personal rankings for certain attributes I find important.
It is in this format: 1st place / 2nd place / 3rd place
24-70 / 24-105 / 17-55
The 17-55 doesn't feel super cheap like the 18-55 or 50 1.8, but it definitely isn't as nice as the L's. The 24-105 feels solid, but the 24-70 feels like a tank... like you can almost use it as a car bumper.
17-55 / 24-70 / 24-105
The 17-55 is sharp in the centre through to the corners, at ALL apertures and at ALL focal lengths. There is NO optical weak spot on this lens in terms of sharpness. IT does flare more readily but it's easy to control, and even fun to use artisitcally. I find the other lenses show weaker sharpness at 24mm, and the 24-105 is weaker at 105mm. The 17-55 yields slightly cooler images. But in all honestly, ALL of the lenses deliver some stellar image quality. It should be the least of your concerns when choosing one of these lenses. There are some who say the bokeh isn't fantastic on the 24-105, to which I would agree, but it isn't absolutely beautiful on the other lenses either. On a non-bokeh friendly scene with lots of high contrast lines and jagged edges that are not TOO far off the DOF, none of them seem too particularly outstanding in comparison to good primes.
Useful Range on a 1.6 crop for GENERAL shooting:
17-55 / 24-105 / 24-70
Well this is obvious because only one was actually MADE for 1.6 crop as a no-compromise solution. 24mm on a 1.6 crop feels restrictive for indoor shooting, unless you are in an open concept mansion and have tons of room to back up. It loses out on the long end, but I find the 17-20 range very critical and used VERY often. In other words, I find I'm changing lenses a LOT more with a 10-22/24-70 combo over a 17-55/70-200 combo. I personally do not enjoy making many lens changes... it's cumbersome and annoying, and I always seem to miss great shots when fumbling with lens changes. (I wish I had 3 hands)
Flexibility/versatility for GENERAL shooting
17-55 / 24-105 and 24-70
With 2.8 aperture and IS, it's ready for you in broad daylight, or the middle of the night. The 24-105 is slower but with IS can get you by in a pinch in the dark. The 24-70 is faster, but when it's really dark, it's really hard to get a 1/60 shutter speed without a tripod. But it also has an easier time focusing than the 24-105 in the dark. It's really a toss up for second between the 24-105 and 24-70 on this one, but the 17-55 is certainly on top. There is no other lens like it. Many people say you don't NEED IS at short focal lengths. Well, those people have likely never taken beautifully sharp 55mm shots handheld at 1/4 shutter, without leaning on anything. As a note, ALL of them provide nice portrait ability.
24-105 / 24-70 / 17-55
Though plagued with a recall in the beginning, the 24-105 currently seems to have the lowest number of complainers, myself included. The 24-70 in the pro world is often hated for it's inaccurate focusing at the wide end. If you can get a good one then you're set. But there seems to be a significant number of bad ones. (Though certainly not in the majority I assume) Test it's focus accuracy at 24mm. Many reported the same issue... backfocusing at 24mm, which gradually improves through the focal range. By the time you get to 70mm, focus is usually spot on. So test that wide end!
The 17-55 has a weak IS motor and dies after hard use. You have to use it ONLY when needed, and it should last forever. Overuse it and it'll break. I was the first on POTN to report this back in 2006. Many identical cases have popped up since then. I have found a rather consistent way to tell if it's on the way out. Half-press the shutter so IS is activated, and slowly zoom from 17 to 55. If it's starting the dying process, you'll likely see a jump in the viewfinder between 28 and 35mm. If it isn't, then you're good. It's been mostly pros who've subjected the lens to rigorous use that have failed 17-55 IS motors, so the casual shooter may not need to worry. In any case, I suggest any 17-55 user ONLY turn on IS when you actually need it. So in other words, shooting under 1/50 with NO flash and no tripod.
Another issue with the 17-55 is dust. This can be avoided by using a UV or clear filter and threading it quite snugly. I believe it collects dust through the space between the front element and the body. If you put your hand infront of the front element and zoom back and forth, you can feel lots of air getting blown out and sucked in. Putting a filter on there really snug will prevent it from sucking dust through there. I am assuming that it now sucks air through somewhere else that may be filtered with felt or something because I am getting no dust.
17-55 and 24-105 / 24-70
Both the 17-55 and 24-105 tied for this as they are almost the exact same weight and size. They are pleasant to use. The 24-70 is huge and heavy. Some people say only a wimp would complain about the 24-70 weight. Well, I challenge those tough guys to hold up a 30D/24-70/580 combo up for a 16 HOUR shoot. That's how I have to use it, so weight is an important factor. For me, the arms really get tired after the 12th hour passes.
24-70 / 17-55 / 24-105:
As the 24-70 uses a reversed zoom design, where it's fully extended at 24mm and retracted at 70mm, it allows the hood to be effective at ALL focal lengths. This is really nice, though it's huge and goofy looking imo, but a really cool design. The 24-105 hood is optimized for full frame and film, so it's less effective than it could be on a 1.6 or 1.3 crop. The 17-55 hood is totally optimized for 1.6, the only format the lens will fit on. I use the EW-83J hood from the 17-55, and put it on the 24-105 to get better control of stray light, without any vignetting on a 1.6 crop. It even works great on a 1.3, so I would suggest any crop camera/24-105 users give the EW-83J a shot.
17-55 / 24-70 / 24-105
Under poor conditions, the 17-55 and 24-70 both seem to perform similarly. When one can't hit focus, the other can't seem to either. First place goes to the 17-55 due to the speed. It is MUCH faster than the 24-70. The 17-55 can go from infinity to min focus and back to infinity in the time it takes the 24-70 to go from infinity to min focus. Considering that the 17-55 pauses when it hits min focus before it's return trip to infinity, I'd guess the 17-55 rips through it's focusing range at LEAST three times faster than the 24-70 can, all with the same accuracy percentage.
The 24-105 on the other hand performs noticably worse than the other two in darker conditions. There is definitely a difference. That's not to say it's horrible though. The 580's AF assist lamp does WONDERS for it in dark places, so AF actually does reasonably well with it. So I would certainly not say the 24-105 cannot perform well in darker areas. Just have that flash handy. And watch out because the hood can block the AF assist light from reaching your focus points, which is why I often remove hoods when shooting in darker places.
Really, they are all about the same price. All have their advantages that make them WORTH their price, so there is no particular standout in regards to bang for the buck. Do you want the ONLY F/2.8 normal range IS zoom in the world? Do you need extra zoom range and IS? Do you need a fast zoom lens that also works on your 1.3 or FF body? All have their merits and all are worth their price.
Ability to be used as a weapon in a fight:
24-70 / 24-105 and 17-55
Seeing as how the 24-70 is a lot heavier, it would likely perform noticably better than the other two when used as a projectile aimed at one's head. As the 17-55 and 24-105 are the same size and weight, they both tie for second place. Though personally I am more the type to try and get away before fighting, it is nice to know the 24-70 can perform well if backed into a corner with no way out. I would also pick the 24-70 over the other two when placed in a shoulder bag and used as an improvised mace and chain flail.
So that is my review on the three lenses. If I had to choose ONLY ONE it would definitely be the 17-55. But all of them are great lenses in their own right.
17-55 at wide: (colours subdued due to processing style)
17-55 at tele:
24-70 at wide:
24-70 at wide:
24-70 mid zoom:
24-105 at tele:
24-105 at wide