I didn't rush to make a review of this camera, as I wanted to really put it through it's paces first. I won't try to list every feature or go over every bullet point (the above description does a fine job), but instead try to go over a few things which make a big difference to me as a 5D Mark II owner. For some background, I bought an original 5D in 2007, a 5DII in 2008 and have been working with these bodies ever since then. I also have experience with all of the Canon 1-series up through the 1DIII and 1DsIII. I currently log about 60,000 photos per year with the 5D Mark IIs as a professional wedding and portrait photographer. I shoot almost exclusively with fast L prime lenses in my work.
So after a week of solid shooting with the camera, here are the areas which are of note relative to previous 5D bodies:
AF is the elephant in the room here so I'll address it first. Good news, we now have a focusing system worth of it's price point. The AF system here is identical to that in the 1Dx and is THE most sophisticated AF system EVER put in any Canon body. It is superior to that in the 1DV and all bodies before it.
I have tested the AF point in servo and one shot mode with my fastest lenses. Speed, accuracy, and consistency have been exceptional and better than anything I have used before. AF gets the job done with zero drama. NO focus jitter, NO frontfocus, NO backfocus, nothing but near-instant, dead accurate focusing with all of my lenses. Even with my Sigma 85/1.4 (which gives my 5DII bodies absolute fits) is 100% accurate with no jitter on the 5DIII. Center AF point and all peripheral AF points are all usable with fast primes. With the 5DII you just use the center AF point and hope for the best (with often mixed results). You could forget using the outer AF points with fast lenses on previous 5D bodies. That has all changed now.
Just to see how far I could push it, I took my most difficult to focus lens (24/1.4 II), put it on the 5DIII, and tried to focus on my black lab in my dimly lit apartment. At a distance of about 2 feet I would able to lock focus on the dog's eye with the far left AF point at F1.4, 1/40, ISO4000. Think about that. I was able to focus on a black eye on a black dog in a dimly lit apartment at F1.4. The 5DII would have hunted all day long trying to do this, even with it's center AF point.
I could sit here and write a book on how happy this performance makes me. For what I do, if this were the only upgrade from the 5D Mark II, it alone would be worth of the $3500 price tag. That said, there is more...
It's hard to put my finger on exactly what changed, but the 5DIII just feels more substantial. It feels like a chopped down 1-series instead of a buffed up 10 series. The contour of the body has changed to fit your hand better. The rubber is also a new compound which is much grippier than before. The 5DIII feels much better to hold and use than the previous 5D bodies.
I wasn't expecting a big improvement here, but the screen is drop dead gorgeous. The height is about the same, but it's wider than that in the 5DII and fits the aspect of horizontal images perfectly now. The screen itself has better coatings which allow you to see it easier outside. The contrast, viewing angle, color, and saturation have all improved noticeably. It has a very similar look to a high end smartphone screen. This is a substantial upgrade from the 5DII's screen.
Image quality is better than the 5DII, but not substantially so. Let me explain.
The 5DIII now natively amplifies the sensor data to ISO 25,600 whereas the 5DII only natively went to ISO 6400. This means that for anything higher than ISO 6400, the 5DIII is better. In RAW you are looking at an improvement of about 1/2 to 3/4 of a stop at high ISO. At lower ISOs, the noise level is about the same.
JPEG quality has improved much more though. The JPEG engine in this camera is staggeringly good and a solid 2 stops better at controlling noise at high ISO than the 5DII. It strikes the best balance of detail and noise control of any camera on the market right now. Note though that default NR in JPEG mode is fairly strong and that you will generally attain a better "look" from your files with the "low" NR setting.
As an aside, the nasty cross-hatch banding present in the deep shadows of 5DII files is now gone with the Mark III. There is still mild vertical banding, but it's similar to the original 5D and only visible when pushed heavily (3 or more stops).
I don't have any hard data on this, but I'm fully convinced the metering of the 5DIII is better than that of the 5DII. I find myself correcting with exposure compensation MUCH less now with the new body than with the mark II. Shooting with the two side the newfound metering accuracy of the mark III is very obvious. I found the 5DII metering very similar to the original 5D. The new 5DIII is much improved here.
**SPEED AND STORAGE**
Camera startup and operation is near-instant. Shutter lag and mirror blackout is now faster than before and leads to a more instant, responsive feel while shooting. This, combined with the vastly improved AF make for a radically different experience from previous 5D bodies.
Dual memory card slots mean you can now either backup your data to a 2nd slot *OR* you can "span" cards. Spanning means that once one card is full it will automatically swtich to the second card. SUCH a nice feature. I can't tell you how many times my card has filled up at the most inopportune moments and shooting stopped. No more.
Shooting speed is either 3fps or 6fps and the buffer is about 18 frames deep in RAW only with a fast CF card. You can shoot almost indefinitely in JPEG mode without hitting the buffer. For RAW I would recommend a 60MB/s CF card to take full advantage of the CF slot speed. The SD slot is slower, but still capable of about 30MB/s write speed.
The 5D Mark II had a slight magenta color cast. This was easily correctable in post processing and wasn't a huge deal most of the time. I now report that color cast is gone and that the 5DIII's color is much more neutral. Skin tones in general look better due to the more neutral tone.
Additionally I have found auto white balance to be improved over previous 5D models. I've noticed that while post processing I'm having to correct color less with the 5DIII files than the 5DII files. This is very exciting, as it will save me a fair amount of time in post processing. Per usual, all of the cameras struggle under tungsten lighting. However, AWB is able to get color surprisingly close with anything that contains natural lighting.
I would strongly advise reading the manual because there are a lot of new settings and options which won't be familar to 5DII users. There are also a LOT of different ways to set up your AF system, so a little experimentation is needed. In general, the menu system is more complicated that before, but this also allows a much greater degree of customization of the camera. In that regard, the 5DIII is much closer to a 1-series than before. Take the time to learn it and set it up correctly.
You now have the option to one-click zoom to 100% at your AF point. This means you can instantly check focusing with one button push. This saves a lot of time and frustration while shooting. There is also a "silent" shutter mode which only makes about 1/2 the noise as the standard shutter. You can do one-shot or 3FPS in silent shutter mode. 6FPS continuous is only available with the standard shutter mode.
Another brand new feature that's exciting is the ability to re-map buttons on the camera to perform other functions. The options are very extensive. One in particular I'm excited about is the ability to toggle one-shot with AI-Servo by clicking the DOF preview button (which is now on the right hand side of the camera, in perfect reach of your middle or ring finger). If you are shooting a still subject in one-shot and they start to move, simply push the DOF preview button and you're instantly in AI Servo mode. There is no need to move your hand, or even look away from the viewfinder. When you are done, simply release the button and you're back in one-shot mode.
Canon finally woke up with the 5D Mark III. The completeness of this refresh is hard to overstate, as there is no part of this camera that was left untouched from the Mark II. The overall experience of using the camera has been transformed to an entirely different level. You will be faster, better, and more efficient with a 5D Mark III relative to its predecessors.
The improvements here will most cater to those who shoot in demanding environments which require high ISO and fast, accurate autofocus. Canon basically fixed most every complaint anyone ever had with the 5DII while maintaining the things which made the 5DII great (resolution, image quality, small body).
The price of this body is probably about $500 too high compared to its primary competition - the $3000 Nikon D800, which is likely to annoy some people. Though individually they cater to different types of photographers and have different strengths over the other, overall these two cameras are comparable products. If you are starting from scratch or have minimal gear investment, the D800 is worth a hard look at. If you are heavily invested in one system or another, you would probably do best just to stick with your current brand. Both are fine cameras and you can't go wrong with either one.