This recording is excellent through and through. At first, I was a bit thrown off at how quiet the timpani solo / fate rhythm motif (which coincides with the major/minor seal) was in the first movement's exposition, but I think it's intentional. During the recapitulation that rhythmic motif is much louder, and any other time it appears throughout the symphony it is prominent. I think Fischer's idea here is that the rhythm gains prominence as the symphony goes on... or as inevitable fate creeps in.
Like Barry I don't care for the third hammer blow, but this is the third recording I've ever been convinced of it. Bernstein's DG recording puts it in a very clever and convincing spot, and Simone Young's is such a tragic reading that it works as well, in my opinion. But convincing or not, I'm always reminded of why Mahler removed it: it's really just there. It hardly, if at all, adds anything to the music at that moment. If there is ever to be an "official" third hammer blow, it should be where Lenny put it in the DG recording.
All in all, I'm happy to say that I believe this M6 is one of the highlights of this very good cycle. The key recordings to listen to, in my opinion, are M1, M3, M4, M6, and Das Lied. The rest are all at least good. I personally don't detect a dud in this cycle, but as we all know there are some that feel very differently.