When done well, very fun! ...but it's kind of more complicated than that.
(you didn't ask for this, but HUGE RANT ABOUT VILLAINESS STORIES INCOMING BECAUSE I HAVE BEEN NECK DEEP IN THAT GENRE FOREVER)
Imo, the fun of it came from its premise of 'turning the cliche (tropey) story on its head to unexpected consequences.' A huge meta-play on the basic storyline. It's kind of like how people fine the butterfly effect and other time travel stories interesting-- how can different decisions change the predetermined future? And how will the main character manipulate the story to their own desired ending?
The best stories came from the ones where the reincarnated villainesses had a distinct personality/goal and made good use of their resources/second chance/already learned information to masterfully play up the game.
The worst stories came from the ones that copied the good ones but forgot to give their heroines a personality, which is why as people flocked to this genre, it became less and less fun. In other words, the genre itself had so much possibilities/potential, but the generic heroine character (you know the one-- dense, klutzy, but somehow charming, doesn't speak their mind, 'smart' but actually doesn't do anything, is always confused as to why stuff didn't go their way when all the signs were already there, etc. etc.) began showing up into the 'villainess' characters and rendered it a complete shoujo mess. And since these new majority writers half-heartedly followed the newfound tropes of this genre (ooh, everyone falls for the villainess instead!), the genre kind of cemented itself into being 'another generic common shoujo trope storyline thingy.'
And I say this with love, because I love otome game reincarnation and shoujo. However, I can't help but hate the weak, generic shoujo heroine as well as unoriginal stories, and that goes for every genre.
So yeah, the villainess genre in itself is great. It has so. much. potential. Like come on. Time travel. Manipulation of information. A heroine who is both usually materialistically superior but story-wise inferior because they're still in a place of danger (both the op protag + the underdog story rolled into one). The sheer meta that becomes possible. Strategy games. Cathartic plot twists. COME ON.
tl;dr: villainess stories are the bomb when they aren't treated like self-insert frames for yet another boring heroine. The character makes the story, and a plotless, personality-less story that runs through the same tropes will probably be boring everywhere, regardless of genre, but the villainess trope got the worse end of that stick imo because it had such a huge boom.
And now I'm kind of mad again thinking of all the villainess stories I've already read and got mad over. :D