We did this route the other way round in January... from Bamako the road is sealed as far as about Didieni (as far as I remember) then it's a badly graded dirt road with bone shaking corrugations as far as Diema. No problem with a VW bus, so long as it holds together. After Diema the road is really rough - deep dusty sandy sections, ruts, basically fallen apart. No problem in anything with decent ground clearance and a bit of power to get through the crap. That's as far as Nioro, after which it's good. Into Mauritania the road is partly brand new - we didn't even see a border post, it's that new. Other bits are under construction, or ordinary dirt road. From Ayoun the Route D'espoir is in excellent condition. Have a good trip - the only vehicles we saw were donkey carts and lorries converted into local buses. I guess most people probably take the other route and go via Nema - we didn't, so I can't tell you what the road is like that way.