When going on vacation and visiting my family back home, I had the opportunity to use and test my father's new Canon 5D MkIII camera which he bought as upgrade of his cropped-sensor based 40D camera model. I read many reviews about the 5D MkIII, but nothing is better to explore myself this camera and get my own opinion if it is worth to upgrade especially when you already own the predecessor model. I won't go into too much details, but will point to some major pros and cons which I became aware of when using the MkIII in comparison to my two existing MkII full frame models.
AF: the biggest strength of the MkIII. The camera offers many options to select areas or single AF points out of 61. The AF system is copied from the professional 1D series obviously. I tested it in combination with the Canon 50/1.4 EF lens, and was surprised to see that even without previous microfocus adjustment and in difficult light situations the center of the focus was edge on at f/1.4. One shot was enough to get it perfect - something which was very hard to accomplish with the 9 AF point system of the MkII. I also like that the 5D III has AF points in the corner of the frame which is ideal for portraits. I did not test the AI Servo mode, but I believe that it tracks much better moving objects than the 5D MkII does.
Biggest drawback of the 5D MkII was its limited AF capability which was exactly the same as in the older 5D classic. Now we got an excellent AF in the MkIII, but....the main drawback is the lack of new sensor development! This is where Canon cut back in this model - for some it is not a big issue who don't want to deal with larger file sizes or just with more MP. Others like me miss an upgrade of a new sensor, something similar to the Nikon/Sony Exmor sensor which has better resolution and higher dynamic range. I would love to have bigger files to be able to crop photos in case of wildlife shots on full frame for example. It didn't happen, for me it was disappointing news when I saw the sensor specs of the MkIII. Still the MkIII comes with new Digic5+ processors for more processing power. I am sure some processing algorithms also were improved compared to the MkII. But other than this, the IQ (especially at lower and normal ISO numbers) is the same as in the MkII. RAW file sizes are about the same as in my MkII what is expected.
Ergonomics: I added this point underneath the cons because one major drawback compared to the MkII is the way to magnify photos on the camera screen - in the MkIII you have to press the zoom function on the left camera side first and then use the wheel on the right top camera side to zoom in or out. Something you get used to, but I didn't see it as an ergonomic improvement. The MkIII was sitting in my hand as well as the bit smaller MkII body - both are equal in handling otherwise.
Price tag: the camera is simply expensive. It has come down now a bit to more reasonable numbers, many purchase it in the US via the Canon refurbished program. Some justify the price because of its professional AF system, but since it is copied from the 1D series, and I can't see why it makes this camera so expensive.
Overall the 5D MkIII would be my dream camera if it had a better/newer sensor in it which I consider the heart of any digital camera. I am not a wedding photographer or photo journalist who relies on quick and accurate AF. The expensive price tag makes it a bit unattractive against other options out there - the entry model full frame 6D camera for example or compared against the Nikon D800E for example. It really depends on the main usage of your camera(s) and your style in photography - I am more a scenery, landscape, architecture, macro, and fine art photographer, AF is just not so important to me. I decided to stick to my MkII camera models until Canon hopefully announces a successor model with (much) better sensor.