To me, the Nexus is actually an amazingly interesting idea that is also amazingly difficult to write into a narrative. It is essentially a "rip" in the multiverse, that allows anyone who enters it unlimited access to the infinite universes and possibilities across all of time and space.
What the concept of the movie (sort of) demonstrates is that such access essentially makes "life" meaningless, though. Without mortality, and with the ability to go anywhere, anytime, with any setting and history...it's even more of a fantasy than a holodeck, because even with a holodeck you are mortal and need to return to "real life." In the Nexus, you have neither grounding elements. I'd imagine that it is one of those experiences that, if you were only there for a limited amount of time, would seem heavenly...but to exist in that manner for all eternity could be downright hellish.
What the concept of the movie (sort of) demonstrates is that such access essentially makes "life" meaningless, though. Without mortality, and with the ability to go anywhere, anytime, with any setting and history...it's even more of a fantasy than a holodeck, because even with a holodeck you are mortal and need to return to "real life." In the Nexus, you have neither grounding elements. I'd imagine that it is one of those experiences that, if you were only there for a limited amount of time, would seem heavenly...but to exist in that manner for all eternity could be downright hellish.