Independent organizations who assess performances of cameras usually share the results that they get from tests of ISO performance (noise level), continuous shooting speed, sensor capacity etc. But one day when your camera stops working, facts that it has 50 megapixels, being able to take 100 frames of pictures, and its ability to take shiny photos of ISO12800 do not matter. If I am in a hurry, I prefer a Fiat that can move to a Ferrari with a flat tire.
I have been using Canon 5d Mark II and Mark III since 2009. In the recent years, I found that chance to test it under harsh natural conditions.
I first used the camera in North African Sahara desert. Deserts are too hot in the day and too cold in the night. And sometimes, the difference between day and night temperatures may reach 50 degrees. These differences in temperature results in expansions and decreases to reduce functionality of mechanical parts. The sand desert is not innocent as it seems. A light wind spreads it all around. After spending some time in the desert, you will notice that your clothes and interior parts of bags covered in sand. In such an atmosphere, contamination of a sensitive machine with sand may results in situations where reparation may not be conducted easily. I have encountered no problems either during or after my photo shoots with Canon 5D Mark II in the desert.
Then I have been to Çıldır Lake in Kars, Turkey. The temperature was as low as -25 degrees. Muradiye Waterfall and Çıldır Lake was completely frozen. I was not able to use the camera with gloves, but I could also last a longest of 1 minute without wearing gloves. The steam from my mouth as I tried to look through viewfinder was creating a layer of ice on the back LCD screen and keypad. The cellphone I had in my pocket was not functioning as the batteries were disrupted due to extreme cold. I was able to take my pictures with Canon 5D Mark II without experiencing any trouble.
I had to use the camera under heavy rain and at 90% humidity for a week in Havana, Cuba. Although I have never dropped the machine in the water, it was dripping wet due to humidity and rain. Even under these conditions, the camera has never even chugged.
As a result to these, I can easily say that although it is an old model, it was one of the cameras with highest performance/price ratio of its time.
Finally, I want to mention a few more points. I am not a camera fetishist, yet I respect the engineers and designers who manufactured this cameras deeply. Besides, based on my results, you can encounter an undesired situation when you use your machine unproficiently. Beware!