Here is the return of my experience of the day on the slopes of the Bois des Rêves!
Positive points: The tracks are surprisingly technical, it is surprising in our regions. We are clearly on typical enduro tracks of intermediate/confirmed level. They are entirely reserved for mountain biking, so you won't risk running into a hiker in the middle of your descent! In addition we have grip everywhere since the ground is slightly sandy. The difference in level is quite significant because the wood is located on a pronounced hillock. There are a lot of different tracks in a very concentrated place. There are jumps, wooden structures, etc... Finally, there are some gentler tracks to go up the slope quietly and start again. There are large car parks nearby if you come by car.
The negatives: The place is small so inevitably the descents are very short and do not allow you to gain a lot of speed. Access is totally free but suddenly it lacks regular maintenance and markup. By dint of passages, most of the tracks reveal a lot of roots and not small ones! So you have to like riding on it otherwise go your way.
Finally I was there for the first time on a Tuesday afternoon and only met a mountain biker but I guess it must be complicated to have the way free on the weekend given the narrowness of the place and the tracks. Also there are quite a few crossings which can be dangerous if crowded.
In any case, do not hesitate to go there!