I test drove an X reg 311CDI with 78k on the clock (for sale at a very cheap price), and really didn't like it. Didn't seem nearly as torquey low down in the rev range as everyone seems to make out, hit the limiter at a very precise 3500rpm, gearchange was stiff, generally not particularly good to drive, certainly not anywhere near the improvment on my frogmobile van that I was hoping for. I've also driven a mate's 03 reg Renault Trafic dCI-100 and was very impressed with the ride quality, handling and general pull of the engine. They're very expensive though (though I'd go for a Master dCI though, can't justify spending extra on an old shape Master dCI that'll be exactly the same as my current van bar the engine, and the new model's just come out, so I'll stick with mine for a while).
Learned a lesson though - I drove up to near Pontefract to have a look at the Sprinter, and whilst it was in very good condition (all original paintwork, real honest condition, definitely hadn't been patched up like a lot of them) I just didn't like it. I could have been Andy Pipkin from Little Britain.
Peter