Yes, it's a funny thing: audiences love hearing the work in live performances, but music know-it-all's remain critical of it. I suggest that you seek out Sir Donald Tovey's essay on Mahler 8, who points out that it's an incredibly well put-together work, obscured only by the amount of noise that it makes. Deryck Cooke's essay is quite helpful as well.
Cooke points out that in M8, the ecclesiastical text receives the march treatment, while the more secular text (Goethe) gets the chorale treatment. An interesting twist.
De la Grange, who makes no claim to being a great musician, loves M8 very much. So do I. M8 is, however, dependent on getting the 'logistics' right, as well as needing some decent singers. Still, its impact in concert can be quite 'indescribable'.
Barry