I ran the Serres last year and will be back in 2014 to make a better job of it I hope!
Terrain is varied. I would put the event smack bang in the middle between fast desert rally and slow national enduro.
Here is a link for a short you tube photo montage from last year. You can get an idea of the terrain from that.
Also, if you search you tube for the Serres 2012 event you will find a lot of footage from when the guys per-ran and finalised the course. Very useful.