After dreaming about this lens for nearly a year, I finally got one and would love to share my thoughts. Since many reviews online are incredibly technical and complicated, I'll try keeping this as easy-to-understand as possible. If you have questions, please feel free to add comments and I'll do the best I can to answer them.
OVERALL IMPRESSIONS
This lens is absolutely amazing. Ignoring its steep price, large size, and heavy weight to focus solely on the photos it produces, I am incredibly impressed. The images are remarkably sharp, have accurate color rendition, and great contrast. I'm using this primarily on a Canon Rebel T2i, and expect that the results would be even more impressive on a full-frame camera such as the 5D series or 1D/1Ds series (aside from vignetting which is apparently somewhat pronounced on full-frame). If money isn't an issue, stop reading this review and order this lens immediately!
WHAT'S INCLUDED
In the box you get the lens, a lens pouch, a lens hood, and a tripod collar. The benefit of the tripod collar is that you can mount this lens directly onto your tripod (instead of mounting your camera body); this allows you to shift between portrait and landscape orientation easily.
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PROS AND CONS
Overall, this is a great lens. There are some inconveniences, but these are far outweighed by the benefits of the lens.
PROS
1) Fast. f/2.8 is great for low-light shots that you simply wouldn't be able to get at f/4 or another smaller aperture. This is great for action photography, such as sports or wildlife. Even if you decide to add a 1.4x teleconverter - which will cause you to lose a stop of light - starting off with a wide aperture of f/2.8, you can reach 280mm at f/4.
2) Very fast and silent auto-focus. This is very convenient, especially in lower light situations.
3) Superb image quality. Very sharp, even crops have great detail! This is an ideal lens for portraits due to its image quality, and the long focal range combined with the wide aperture make for some great background blurring.
4) Great Build quality...non-L lenses feel like toys by comparison. If your gear takes a beating, strong build quality is a must. Even if you're gentle with your equipment, it's reassuring to know how tough the construction is.
5) Great IS. Generally, one can ensure a steady shot at the reciprocal of the focal length. Say you're shooting at 70mm, you should shoot at 1/70 second or better on a full-frame sensor. On the APS-C sensor bodies (Rebel line, xxD line, 7D), you have to factor in the 1.6 crop factor, so at 70mm you should in reality be shooting with a shutter speed faster than 1/(70x1.6), or 1/112 second. With four-stop image stabilization, you can hand-hold at much slower speeds than with the non-IS version, allowing you to capture crisp pictures that wouldn't otherwise be possible.
6) Weather Sealing. The IS versions of both the f/4 and f/2.8 Canon 70-200 lenses feature weather sealing, and the non-IS versions don't. If you don't have a weather-resistant camera body (i.e. Rebels, xxD series), this shouldn't make too much of a difference, unless you hope to someday upgrade to a more rugged camera body.
CONS
1) Very expensive, but if you have the money, most certainly worth the investment. If you want to save money you can go with an alternative version of a Canon 70-200 or go with a third-party option (see above).
2) The lens is huge...so huge, in fact, that a photojournalist was killed in 2007 by forces who mistakenly thought his 70-200 2.8 (Mk I) was an RPG. If you don't want such a huge lens but want the range, you can either get the smaller-aperture 70-200 f/4, or the recently released 70-300L.
3) Very heavy. People aren't exaggerating, this lens is heavy to handhold for long periods of time. On a positive note, your photos won't only get better, but your arms will get stronger ;)
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COMPARED TO ALTERNATIVES
Canon currently has four 70-200 lenses, two with an aperture of f/4 (one of which features IS), and two with an aperture of f/2.8 (one with IS). The old Mark I version of this lens can also easily be found used online.
70-200 f/4 Series (IS and non-IS): The lens being reviewed has an aperture of f/2.8, which lets in twice as much light as a lens with an aperture of f/4. I settled for the slower (smaller-aperture) 24-105 f/4 over the 24-70 f/2.8 due to the IS, but I do at times feel its limitations which is why I wanted f/2.8 for telephoto. If you don't need the wider aperture (i.e. will always be using an external flash, or will mostly be shooting outside in adequate light), the f/4 lenses are sharp and very popular options. If you plan to use your telephoto for action shooting (i.e. sports, wildlife), you're better off with an aperture of f/2.8 so that you can use fast shutter speeds.
70-200 f/2.8 non-IS: Image Stabilization is a luxury that you pay a premium for, but in my opinion it is worth it. While it won't freeze action, it will reduce the photographer's camera shake and make many more images usable. If you have proper form while shooting you might argue that you don't need IS, which is fair. But even if you take precautions to minimize camera shake, it is at times inevitable, and IS really helps in those situations. If you have the money for it, I'd spring for the IS version.
70-200 f/2.8 IS Mark I: I don't own the Mark I, but I have rented it on several occasions and definitely see what others have been talking about this lens being sharper. That being said, I was willing to buy the Mark I but see that people are selling the Mark I used for nearly as much as the Mark II version costs new! It's worth spending a little more for increased sharpness, improved optical quality, and better image stabilization.
Third-party lenses: Manufacturers such as Sigma and Tamron also offer fast 70-200 telephotos, and since I don't have firsthand experience with them, they might be great alternatives for those on a tight budget. Most of the professional photographers I know, however, shoot with Canon lenses, which might say something. If you can afford the Canon lens, go for it as most thorough reviews with extensive testing frequently show they offer superior optical quality, even though third-party offerings might offer more bang for the buck.
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A NOTE FOR REBEL USERS
I was using my Rebel with a battery grip in order to add counter-weight when using my other L lens (24-105), because I feared such a heavy lens on such a small body would cause my camera to tip over. After using a Meike knock-off of the Canon battery grip for half a year with no problems, I yesterday noticed I must have screwed my grip on too tight, because it wouldn't come off no matter how hard I tried. After Googling this I see that other people have had similar problems with battery grips, even the genuine Canon one. Fortunately I did manage to take it off after following some tips I found online, but I probably won't be reattaching it to be safe (or I'll at least be incredibly careful not to screw it in too tight!). Using the 70-200 f/2.8 IS II with a light camera like a Rebel without a battery grip has been fine so far, so don't feel like you need to get one just to more evenly distribute the weight.
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RECOMMENDED ACCESSORIES
1. A good filter. I highly recommend you put on a high-quality filter from B+W, Hoya, or another reputable manufacturer on this lens...slapping a cheap filter on such an expensive lens would degrade your image quality, ultimately defeating the purpose of buying such an expensive lens to begin with. I like to keep a UV filter on the lens at all times since it is a great way to protect the front of the lens from getting scratched.
2. A good bag. If I want to lug several flashes, bodies, and lenses, I use the Lowepro AW200 which can easily fit this, but you might have a tougher time finding a suitable holster-style bag. I now use the
Lowepro Toploader Pro 75 AW, which can fit this lens (with hood reversed) on a gripped body.
3. A sturdy tripod. This lens is heavy, so you might be pushing the weight limit of your current tripod. I personally use an
Induro AKB0 Tripod Kit (Black), and it's held up great so far. Additionally, I've been using a
Joby GP8-BHEN Gorillapod Focus with Ballhead X bundle as a secondary tripod and a pseudo shoulder-rig for video.
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CONCLUSION
Making a decision on whether to buy such an expensive lens is not easy, but the 70-200 f/2.8K IS II's benefits far outweigh its inconveniences. If you're looking for a great telephoto lens that can quickly capture sharp images - even in low light - this is worth every penny, and I highly recommend it.