Slowly but surely, Chicago are being elevated out of the middle-of-the-road kitsch bracket - more people are discovering their early work, much of which couldn't be further removed from Hard Habit To Break et cetera.
In the early '70s, they called this sort of music "brass rock" - a term that should be revived, as it certainly isn't jazz-rock fusion, it's not a bit like the Mahavishnu Orchestra, twin-guitar '70s Miles Davis, early (Vitous-era) Weather Report, or Frank Zappa's instrumental-oriented stuff. It's closer to Blood Sweat & Tears and such lesser-known phenomena as the Ides Of March or Heaven (who may even have coined the "brass-rock" term on their one and only [great] album).
Or Santana - because even before the Latin percussion starts creeping in on Volume VII, Chicago were mixing the same influences in the same fashion, and they had their own guitar hero in the perenially-underrated Terry Kath (odd that, since Hendrix was quite bowled over by him). I would recommend this to curious Santana fans, but I feel I may be wasting my time. To most people, this is an incomprehensible, utterly time-locked, musical genre. (I've seen reviews of both early Chicago and BS&T which argue that this music communicates absolutely nothing to later generations, and they may be right).
Chicago Transit Authority (1969) is the most guitar-heavy album of their career, with a lot of time taken up by Cream-ish extended jams (Poem 52 and the epic Liberation) and a Hendrixy atonal noise solo (Free Form Guitar. which doesn't really have enough variety of tones and ideas: Larry Coryell would've done it better). But it also introduces the mix of straight pop, jazz-tinged pop-rock and hippie politics that would occupy the next few albums.
Chicago (1970) is the most hippie-political album of their career, with a "stop the Vietnam war" song-sequence toward the end, songs about benign acid trips (Fancy Colours) and groupie encounters (The Road), interracial romances (at least I think that's the subtext of the famous Ballet For A Girl In Buchanon - two excerpts from which became hit singles) and a sleeve dedication about "the Revolution in all its forms". Oh yes, 25 Or 6 To 4 - by far the best-known song from Chicago's interesting years - is included here.
Chicago V (1972) sees them backtracking a bit - in State Of The Union they're anxious to distance themselves from the anarchist element, even though the song describes an instance of oppressive policing, and Dialogue is ambiguous as to whether we're supposed to sympathise with the "revolutionary" or the "apathetic student" depicted in the lyric. The songs are shorter, with no extended solos or ambitious bits of arranging. At worst, though, it's still a charming early '70s artifact - and the best bits remind us that they were still serious musicians. A Hit By Varese obviously stands out - as does the unfinished bonus track A Song For Richard And His Friends, which unfortunately isn't here (you have to get the separate-disc edition!)
Chicago VI (1973) is their first straight pop record - the beginning of the transition towards AOR and MOR. At least it's a nice guilty-pleasure, much like the early Billy Joel or Steely Dan albums. Terry Kath is still on hand to add a bit of vocal grit and guitar muscle where needed, and the gospel-tinged finale Feeling Stronger Every Day is irresistible. (But again, it's better to get it as a separate disc, for the brilliant Terry Kath outtake Beyond All Our Sorrows, not included here).
Chicago VII (1974) is mainly another very '70s adult-pop record, like its predecessor, but it also contains the last gasp of their progressive, musicianly aspiriations. It opens with an instrumental suite including some damn fine flute, guitar and drum solos, and there are other, more fragmentary instrumentals punctuating the record (Hanky Panky and Mongo-Nucleosis could and should both be taken into the standard jazz repertoire, with minor modifications). Of the vocal numbers, Kath and Lamm's contributions stand out, but there are unexpected bonuses in the famous Beach Boys collaboration Wishing You Were Here, and Jimmy Pankow's vocal Song Of The Everglades, which features some splendid Kath guitarwork.
There's an obvious problem with this box set - Chicago III is missing, which is nearly unforgivable! It may not have yielded any hits, here or there, but it's the most musically ambitious, most consistently compelling album they ever made. The one album that could definitely convert a skeptic: in its way the only Chicago album you really need.
So buy that one first, and then if you want more, think about buying this. (But bear the "no bonus tracks" situation in mind!)