I have many reasons to like many RPGs, and Neptunia has several things that makes it a great series.
One, it is delightfully meta without being too cliche. The plots reference all sorts of video game history, memes, and injokes to the point you know the writers did their homework.
Second, as a game dev, the engine has gone through many tweaks yet the core seems very economical and streamlined, not to mention quite intuitive, with hardly a pause between games aside from time spent figuring out the changed mechanics.
And finally, it's FUN. And if a game doesn't have that going for it, why bother, right?