I'm late to this M7 party, but count me in as a HUGE FANATIC of this piece!!!! During my Mahler "dating" period, I did M5, M6, and M7 in that order, and by the time I got to M7, I knew I was a lifetime memberr. It snowballed from there.
To my ears, his orchestration in M7 is hard to beat - and I think his only other symph that gives it a run for its money in its architectural craftsmanship is M9, but that is me. The first movement just kills me - it is just pure genius. Listen to the harmonic structure ONLY sometime - it will blow you away. And we ain't even talking about the counterpoint, melodies, themes, structure, etc , etc yet.
I own a bunch of M7's and I like all of them for different reasons but nobody has mentioned Boulez yet. It is not a perfect recording overall, but I have yet to find any conductor NAIL the middle section of mvt. I where the harps do a glissando that melts into trilling flutes. THat part always reminds me of some fairy tale castle (yes sounds corny) but makes me feel like a kid again. Boulez in his relentless perfectionism just owns small details such as this part - most conductors just gloss over this detail. I'm sure Mahler would smile if he heard that Boulez recording because the stuff printed on the page comes off.
The next one is the one I still struggle with (M8).