I often travel to Troisdorf for business and this has become my "standard" restaurant.
The food is always good and nicely presented, the portions generous, and the price quite reasonable. One negative point is that the menu hasn't changed a bit in the last 9 months, however every week they offer a few different seasonal dishes (presented on a blackboard).
The personnel is in general very friendly, although one woman (I guess the owner or co-owner of the restaurant) was initially pretty cold and businesslike in her interactions. Basically she came with the check, got my credit card, came back with the receipt, and all the while barely said a word. With time she got friendlier.
I would certainly not call this a "gourmet restaurant": they have nothing fancy on the menu. However if you want a good meal in a comfortable place, you can find it here.