I saw a piece online somewhere that argued both sides but came down on she was real, and made interesting points. While K clearly decides that it was a lie what with the billboard Joi saying "Joe," and then deciding to go [blackout]give his life to save Deckard's[/blackout], Joi very well could just be the next generation where artificial intelligence is giving life to artificial intelligence. And just because the Replicants view her as lesser, it raises a question of whether they're any less misguided than the humans like Robin Wright who think K (or Roy Batty) have no souls?
And it comes down to her choosing to let herself be deleted off the console. That isn't what K wanted to hear, but she insisted anyway, so I am very open to her being sentient, even if she is less independent or self-aware as K is.