First of all, I need to stress that this is my no means a technical review of the Canon 5D Mark 3. I will leave that to the experts. I cannot compare this to other brands such as Canon's main competition Nikon because I have never used one. I will however compare this to my 7D. I have also had the opportunity over the last few years to work with the 5D Mark 2.
The first thing I noticed when holding it, is that the memory card door is rubberized, so that the entire grip feels uniform all the way through. Nice touch Canon. While I'm talking about the memory card door, underneath this door is TWO card slots. One for a Compact Flash card, and another for an SD card. Now I can try that Eye-Fi card I've always wanted to try. More on that another day...once I get one.
Due to the fact that the 5D Mark 3 and the 7D both use the same battery, I didn't have to wait for the battery to charge before taking it for a spin.
With my first click of the shutter, I heard myself whisper quietly with a "Niiiice!". I know that the sound of the shutter means nothing regarding image quality, but hey, I'm here to give you my thoughts on this camera, and that's what you're going to get.
The auto focus has been taken from it's big brother, the Canon 1Dx. Not everything from it, but enough to give the 5D Mark 3 an incredible auto focus system. With 61 auto focus points, you should always be able to frame each shot perfectly. This camera focuses extremely fast on any of these points. In my limited time to test this so far, even under the worst lighting conditions, it locks focus in no time at all.
As I mentioned, I've used the 5D Mark 2 several times in the past, so I knew what to expect with my first full frame camera. If you've never shot with one, you'll love how much more 'image' you get with a full frame sensor.
The 5D Mark 3 is a huge upgrade from the Mark 2, and a massive upgrade from the 7D. It is very responsive, and to put it in real life terms, it acts like a camera should. It does what a camera should. I have no doubt about it's abilities. I feel very confident working in any conditions with this.