I'm sure you're quite familiar with the original album, so I'll skip those discs and focus on the 64 tracks of additional material.
The quality of the polished tracks that didn't make the double album release goes to show just how deep the quality of the material was.
Even the purely instrumental tracks are strong. Despite having no lyrics, "Feelium" almost sings to you. The bold and fuzzy "Glamey Glamey" is gripping from the first note to the last. "Bagpipes Drone" offers up something you'd expect from The Velvet Underground rather than Smashing Pumpkins.
The alternate mixes are hit and miss, as is to be expected. As such they account for many of the compilation's weakest moments. Among the most disappointing is "Zero (Synth Mix)", a cut who's synths are not nearly as prominent as the title would suggest. Not much better is "By Starlight (Flood Rough)" a listless run through of one of Mellon Collie's most under-rated songs.
Other takes, like the more drum-focused "1979 (Sadlands Demo)" offer interesting new perspectives on familiar songs. The two "Tonight, Tonight" tracks, a string-only mix and a band-only version, offer great insight through a deconstruction of that hit.
Sometimes there are good reasons for the material not making the final record. For the sluggish "X.Y.U. (Take 11)" and the meandering "Thru the Eyes of Ruby (Take 7)" the reason was a superior version. With the bloated "The Aeroplane Flies High (Turns Left, Looks Right)" it just isn't a very good song. In cases like the jaunty and fun "Autumn Nocturne", and the twangy "Fun Time" it is because the song just didn't fit the mood of the overall record. The cover of Joy Division's "Isolation" gives a glimpse of the more electronic sound that was to come from the Pumpkins.
Yes, Mellon Collie & the Infinite Sadness box set is a slog, and yes there are a number of tracks that were justly thrown away back in 1995. However, the sheer volume of material, and the surprisingly strong overall quality make this a treasure trove for Pumpkins fans.