One of THE gold standards of albums in terms of content, musicianship, mastering and mixing, and even their weirdness/quirky noir-ness down to the now famous cover (oddly enough taken by Phil Hartman). Very much has evolved into a album synonymous with a generation and as a result of this, most people in my age group either love this band, or really passionately despise it. Give it a go. I hated them once, too.
As far as what they're about well...it's a very clever crafting of old jazz, bits of R&B, fusion (in a not over-the-top way), newer jazz of the time, hints of pop, use of large chord extensions, rock, funk, blues...all melting around in this duo concoction. These guys "shopped for their talent" in the studio. As a result, the album credits for this and most of this era's work from them has very lengthy personnel listings--one song was done, and effectively, a "new band" of musicians would be slated to play on the next cut of the album. Perfectionists to the nth degree. However, the results are pretty damn impressive and remain so now even 40+ years on. Effectively, they were THE industry's Dark Jazz-Rock ensemble of the "best of the best" for 7 years back in the 70s. Few others in the genre achieved this niche of Jazz/Rock success as well as this band has with exception to perhaps Weather Report.