In reply to dinkypen:
I sometimes use some of it as my route to work!
Around the eastern end it's mostly fireroads through the vineyards and garrigue - very pretty but not particularly challenging. The best riding around Montpellier is not waymarked (but can be found on GPS sites and increasingly on Strava).
After La Caylar on the Larzac plateau it apparently changes to use a lot more singletrack. I haven't ridden much of this part, but I've crossed some of it during the "Larzac Brebis Tour" VTT rando.
I don't really know anything about the western parts, but judging by the leaflets here :
http://www.herault.fr/sports-loisirs/publication/reseau-vert they're much the same. The whole thing is based on the "Reseau Vert" which is intended to be usuable by walkers, mountain bikes and horses so it can't be too technical.
The waymarking is excellent - instead of old, faded and often hidden plastic signs there are green metal posts at all junctions (one before, one after. It's not impossible to get off the route (ahem..) but anyone not going flat out should be OK.
I have occasionally crossed what look like school groups doing sections. With a little imagination it would be possible to do a very civilised multi-day trip staying in a mixture of gites and refuges.
The newer Traversée Larzac-Méditerranée looks a bit more interesting in terms of riding (at least the first half). It's 188km, and gets progressively easier, so in theory it would be possible in one long day:
http://www.herault.fr/files/gcausse/VTT-Larzac-Med.pdf
Actually, the FFC-VTT centres that Larzac-Med crosses through are quite a good example of what I was talking about earlier: The Salagou one is excellent, with some varied and challenging circuits, but the Bessiles one is mostly loops of varying lengths through vineyards and IMO is not really worth travelling to.