I watched it because I enjoy musical theater, but I found it highly overrated. To be honest, I thought it tried too hard to be a "cult" film, which is something that should only come naturally and shouldn't be forced.
The actors singing voices were also sub-par by musical standards, I think only Anthony Stewart Head scraped by due to his prior singing experience on Buffy, brief as it was. I can understand how it's a hit to a certain audience, but to everyone outside that circle, it's not all that great.
And Repo Men did not rip off Repo the Genetic Opera, Repo Men was based on a novel called Repossession Mambo by Eric Garcia.
To be honest, I enjoyed Repo Men much more than Repo the Genetic Opera. In the Genetic Opera I felt like the stereotyped goth/cyber punk style was just being forced down my throat and that the film tried too hard to be "edgy." Repo Men had a dark tone without having to use goth and white-out contact lenses, and the twist ending made it all the more tragic.
tl;dr: I'd watch Repo Men over Repo the Genetic Opera any day of the week.