I also have a Gaggenau combi oven and have just begun using it to cook sous vide. I tested the accuracy of the built in temperature probe against my calibrated thermometer and found it accurate within +/- 1F. Last night I tried the oven to cook steak. I first preheated the oven to 135F. To provide more constant temperature I elected to fill a container with tap water already heated to 130F. I bagged and immersed the steaks in the water having inserted the temperature probe into the center of one. In about 30 to 40 minutes I had reached a core temperature of 129F. A separate min/max thermometer in the water bath had never varied more than one degree F. Cooking had required no oversight whatsoever. After hot searing in a cast iron pan the steaks were perfect. I do, however, plan to get a suitable torch as soon as I can.
Although I am sure I will eventually invest in a Polyscience immersion circulator the oven has greatly reduced the urgency. Moreover, with both oven and circulator I will be able to simultaneously cook multiple items sous vide at different temperatures. The combi oven has a minimum set temperature of only 85F. I have only tested down to 105F but it seems to be accurate at least that low.
