It's the only "later" Sondheim show I genuinely like.
That being said, the score really outweighs the book, which I feel lacks a lot of depth (Clumsy Ford jokes?). The songs do nearly all the dramatic lifting in the show, which is fine... except that some characters rely nearly exclusively on the book scenes for development and psychological insight.
Even in the book depository scene, which is arguably the strongest book scene, things don't get really interesting until music takes over.
However, the score is really great and not a single number is expendable.
I don't begrudge the initial cool reaction to the original production. The musical was a little before its time; exploring the flip side to the American dream is more common now.
"...everyone finally shut up, and the audience could enjoy the beginning of the Anatevka Pogram in peace."