I did watch it, and it´s one of my favorite shows of the 2010s. I still listen to its soundtrack very often and follow the careers of the young talents in it. I have read that the second season isn´t appreciated by some (not me), including the season one actors who were axed. To my surprise, Anjelica Houston recently revealed that she didn´t like filming this at all. She is just amazing as the no-nonsense producer who is always at odds with her ex-husband.
The two young leads were a discovery to me, especially Megan Hilty. She is great here and everything else I have seen her in. I had no idea of who Katharine McPhee was because I don´t watch talent shows, but she manages to be convincing in her role despite her (or Hilty´s) nul resemblance with Marilyn. But this isn´t a Marilyn biopic as you said, but the making off of a musical based on her. Funnily enough, both "Smash" and "Bombshell" eventually became real stage musicals.
The casting is phenomenal, including the smallest roles. The soundtrack is memorable and catchy. Debra Messing knows she is playing a lifetime role that is above "Will and Grace". And what´s more, it´s so soapy and addictive, with no guilt attached. I would recommend it to anyone who loves music and movies! (Don´t expect "Glee" 2.0 despite the appearances!)