this lens is really awesome. if you want three lenses for your camera, get this, the 24-70mm f2.8L, and the 70-200mm f2.8L. They're all perfectly sharp and useful in their own ways. They're especially great if you don't want to have several more prime lenses (and much less money.)
compared to the Mark II verson of this lens, i don't see a huge difference. especially not in sharpness. but don't fool yourself, the newer verson is better. at the same time, the mark I verson (in my opinion) out shoots the 17-35mm, the 17-40mm(some people would say otherwise), and for those EF-S users out there, the 10-22mm. But it all depends. First off, anyone could have any of these lenses and have GREAT results, and someone could use one of these lenses and have terrible images. It all depends on what you're shooting, and how you shoot. If you need a fast L lens that's not killer on your budget, get the 16-35mm. If you have as much money as you want, then get the MkII verson. If you're short on cash, and want a wide angle for something like landscape photography with your 5D then the 17-40mm f4.0 will be perfect. however, i do a lot of photojournal/freelance work, and i simply cannot shoot with f4.0 on a wide angle lens. even with a flash like the 580ex or the 430ex. so instead of going out and saying, "which lens is best?" look at their strong holds, and figure out which feature you like the most. If you're using something like the Rebel XTi, then I highly recomment the 10-22mm F3.5-4.5, but if you plan to use ff any time soon, then don't invest 700 dollars on a lens that will be useless in a couple years. however, if you firmly believe in Canon's EF-S line up, then go crazy. I'd say get the 10-22mm and the 17-55mm F2.8IS. But these aren't L lenses, and they're not the L lens build quality. the 17-55mm is great, but the 16-35mm on a 1.6 cropped body will work just as well if not better. when people come to me and ask, "what lens do you think I should get?" the first thing i say is, "what do you want to use it for?" that's the key. find out exactly what you want from a lens, then find the lens that can do that. don't just try to buy fancy L lenses for the fun of it. you might find that a non-L prime lens might be better for you (and way easier on the wallet)
however, one time a very talented photographer told me that it's not what you're shooting with that matters, but who (or what) you're shooting and why. that's always stuck with me. the material that you shoot is by far the most important thing. you can have all the L lenses on a mkIII you want, but if you're not out there shooting the right things then it doesn't mean a thing. but having a couple nice lenses and a mkIII never hurt anyone. well, your wife might kill you if she finds out how much you're spending. haha.