Well, this one is very special to me for several reasons. First, as a longtime fan of the band, I'd already seen them in concert several times before this album came out. Also, it was known that Mike Pinder had grown dissatisfied with certain aspects of being in the band, wanted to leave, and was about to go (he did, but came back, at least for OCTAVE, then left again for good--and a good call, IMO).
It's also a fine quad mix, one I would showcase to anyone curious about why quad sound was worth investing in. Also, the music is, unlike previous albums, a bit more relaxed overall, as if the band knew they'd gone as far as they were going to get; a summation of what made them unique, but not a lot of flash or flamboyance. And "I'm Just A Singer" was a very obvious statement that they weren't the 'mystical' or pretentious band they'd often been accused of being, but also guys who could rock very hard (and still do!)
And unlike DOFP, it has all the music--no later overdubs missing (AFAIK), no extended passages (that is, it offers less surprises, but is in a way the better for that). And it was their first #1 album in the US. They'd toured here for years, and it paid off in better sales and a lasting, dedicated (if now aging) audience that remains.
ED