To me, his M9 recordings are self-evidently quite great. I like the "live" one better than the studio one, but John Kim - our resident M9 specialist - feels just the opposite. I think his Mahler 5 is quite great too. It's still one of my very favorite M5 recordings ever. I feel less strongly about his M4 & M6. I've never understood why the British critics (Gramophone; BBC Mag.) hold Karajan's Mahler 6 in such high esteem. I really don't get that. But it's certainly not terrible either. Karajan's "DLvdE" is really quite good also.
I think Gramophone and BBC pushed the Karajan Mahler 6 for so long simply because, A.) they never considered the Barbirolli one to be so hot (which I actually like better than his highly celebrated M5), and B.) the old Horenstein/Stockholm M6 wasn't very recommendable because of all the "clams" from the brass. They weren't about to push Bernstein's early Mahler, so they just clung to the Karajan M6 - once that came upon the scene. I actually prefer Haitink's first Concertgebouw M6 recording to the Karajan. I think that Haitink's two inner movements for M6 - back in those days, mind you - are much better than Karajan's.