I have the Kenwood Cooking Chef, and while it's great for making soup and frying onions (it makes a mean French onion soup), I wouldn't use it for sous-vide. The temperature is only measured in the bottom of the bowl, and not in the mass of the water, and the temperature fluctuates quite a bit.
I tried to use it to temper chocolate, setting it at 32°C and stirring until the chocolate was melted, but it solidified on the edges and got too hot in the middle.
It's a very solid mixer, and the option to add some heat comes in quite useful more often than I had imagined. For example, when making bread in a hurry, I frequently turn up the thermostat to ~30°C while mixing, to speed up the yeast.