I had the original vinyl recording of COS and this was a replacement to a damaged CD version I had also bought.
Rush's first two albums were very rock-like, but COS marked the band's first foray into conceptual work. As I understand things, their label wasn't too impressed and wanted them to change it, but it has stood the test of time and marks the point where Rush turned from a normal rock band into a unique one.
The opening track Bastille Day was an anthem in its time, with its racing guitar and Rush's favoured stop-time approach back then. The last track on 'side 1' (in vinyl terms) was a mini-story (The Necromancer), with atmospheric sound effects mingling with strong instrumental work.
The remainder of the CD is another - more loosely defined - mini-story. It has a slightly folky feel to it, but shot through with the now-familiar Rush heaviness (though at the time the direction the band would subsequently take was in question).
Listening to any Rush work I find it hard to distinguish between new and old material purely in terms of quality. Rush has always been tight and clinical in their approach to studio albums - but I think COS was probably the one where they put down the rules for all their future work.
Again, this is a part of history, so I would recommend owning COS for that reason alone. Of course, no Rush fan would be without it anyway.