For me (mono partner in a mono/poly relationship), it's been a struggle at times, but my partner and I have a relationship that is worth all the work. That whole gag-inducing "he's my best friend, we finish each other's sentences, we work well together, we can cook and put IKEA furniture together without killing each other" type of thing.
Yeah, it's THAT good.
For him, he's the happiest he's ever been in his entire life.
So yeah... while it's work, it's still good.