I'm quite surprised that 'Subspace Rhapsody' has such a poor overall rating here on IMDB, considering that Buffy the Vampire Slayer's 'Once More With Feeling' is the highest-rated episode of that show.
It is, however, obvious that the writers were not only inspired but also heavily influenced by that episode of BtVS. At the start of 'Once More With Feeling', dramatic tension is at a peak due to the secrets the various characters are keeping; the episode essentially unleashes all the drama in one climax. The showrunners tried something similar with 'Subspace Rhapsody', but ST:SNW is much more episodic than BtVS and doesn't feature quite as much dramatic tension. For this reason, 'Subspace Rhapsody' doesn't manage to reach the pinnacle that was 'Once More With Feeling'. There just aren't that many secrets that can be revealed in ST:SNW. Skipping 'Subspace Rhapsody' doesn't have much of an impact on the overarching plot of the series, whereas 'Once More With Feeling' was perhaps the most essential episode of BtVS' season six.
That aside, 'Subspace Rhapsody' works pretty well. The songs are catchy (though perhaps not quite as brilliant as Whedon's work), the choreography is solid, and the musical numbers fit their characters and setting surprisingly well. The runtime of one full hour is perhaps a bit too long for a musical episode; both Una's and Uhura's solo numbers felt largely unnecessary. Still, I'm glad the ST:SNW isn't afraid of trying new things (similar to the crossover episode we had earlier). I hope they'll keep their courage to experiment for future seasons as well. But I'm also glad that the whole 'Musical Episode' idea can now be taken off Star Trek's bucket list. It's not that well-suited to the format.