I was at the third preview, and absolutely *LOVED* the show.
Yes, the score is bad -- putridly so. There is not a single hummable or memorable song. The score is horrendous and grating on the ears from the first note to the last. And the sound design is such that it is very, very hard to hear many of the lyrics. So you're stuck listening to awful music, and not even understanding how the plot or characters are developed while the songs are screeching in your ears.
But the book is FUNNY. REALLY funny. Maybe you have to be of a certain age (I'm 40 -- same age as most of the characters) to get many of the humorous references to growing up in the 80s and 90s, and to understand the angst the characters feel and exhibit about being this age, at this particular point in time, without having had created a family or fulfilled one's professional dreams. I absolutely adored the humor and identified with the characters. I suppose if you're not the target age, and aren't experiencing the same problems that consume these characters, you may well find it less relatable.
You have to go into a show like this in the right frame of mind. It is not 'high art', as many have pointed out. It is not in the least bit serious (although the driving motivation of the primary characters is, indeed, a substantive longing that, as noted, many can no doubt relate to). It's just good, irreverent, fun. The audience was laughing uproariously throughout -- the only times they were quiet was when they had to listen to the godawful songs. You have to be in the mood to laugh, go with the flow, and accept the (mostly) lowbrow humor for what it is, and a show like this can be very enjoyable.
Most of the actors playing the supporting roles were very adept comic actors. The guy playing the protagonist is, unfortunately, quite bland. A John Cariani or Rob McClure type would have done a much better job in the role.
Speaking of which, the show follows in the footsteps of shows like Something Rotten, Disaster, Honeymoon in Vegas -- all of which I loved...just fun, silly, funny, musical comedy. If that's not your thing -- if the only musicals you enjoy are heavy pieces like Great Comet, Les Mis, or Hamilton (all of which I love, too!) -- then you won't like this. But if you go in ready to smile, ready to laugh, and ready to have a good time, you will love this...just as I, and, it seemed, everyone else in the theater, did. Really a lot of fun.