Yes, we've stayed at the Ibis in Chartres. It's comfortable and clean, in a good location with plenty of parking. We didn't think the restaurant was good, perhaps it was just an off night in the kitchen. And the breakfast seemed pretty sparse for the price.
(I didn't mention it here because the OP was asking about German Ibis hotels. Overall, our best Ibis experience was at the Ibis in suburban Utrecht. Newly renovated rooms, informative, sociable staff. We stayed there a few times, made a good base for exploring towns like Amersfoort)

Thanks for the many comments. I feel better about the choice. We are looking at Ibis hotels in Frankfurt, Heidelberg, and a few other places. Thanks again for the responses.

-Chels

Another endorsement for Ibis, though our experience was back in 1994. We stayed in several (5-6) during a 3-week driving trip from Strasbourg to the Riviera. At that time, there were very few outside of France. They were all exactly the same, clean, very reasonably priced, simple decor with a decent and inexpensive restaurant. They aren't really charming in style but serve as the "moderate" American hotels do. Some were in the centers of the cities, like Avignon, Dijon and were hotels. Others were out of town (more like motels). At that time they all had free parking. We have always said that we would stay at an Ibis if looking for an inexpensive hotel. They have opened so many more in other countries than in France, but I would think they have maintained their standards. Happy travels!

We stayed at an Ibis in Paris. It was a teensy-weensy little room with a teensy-weensy little bathroom, but it was clean and comfortable.

The highlight of our Ibis stay was watching the Olympics in our room and hearing the announcer say, "Ooh, la la," after a beautiful dive by one of the athletes. We thought that was great!

I stayed at the Ibis Star in Copenhagen in May and it was more than fine. The room was like a smaller version of what you'd find in a Fairfield Inn by Marriott - modern with all the basic necesities. The Danish buffet breakfast was really good, and the price - in pricey Copenhagen - was excellent.