In Jan 2004 the bridge was operational, and the main supply route for Mali from Dakar port (lots of Articulated lorries).
Kidira police are in town, near the railway, customs are on the ramp leading up to the bridge.
Mali customs are on the ramp off the bridge, and then you turn left to the railway line to go and see the police (who charge)
With the carnet the customs were perfect, quick and don't charge either side.
The Mali visa was cheaper in Noakchott than in Senegal (can't remember how much)
The road in to Kidira is good, both from the Casamance(really worth a visit) and from Dakar.
Kidira Kayes was really bad, huge undulations evolved from holes the length of a car!
After Kayes if you take the north loop it is beautiful new tar all the way Sandaré-Lakamané-Diangounté until Diema.
Diema -Didieni is 180km of LORRY size corrugations (watch out for the truckers getting their trailers sideways.. terrifying). There is a parallel piste, but it's slowish, it took us 7 hours.
After Didieni the tar's good again.
A friend followed the railway from Kayes to Bamako and said it was adventurous to say the least; more scenic though.
Hope this helps.
Rob, which way did you go?