I remember that Flyable Heart have (MTL) translation patch and that the menu has very long (5 seconds) loading, although the most annoying one would be the need to mount the DVD if you apply the patch, which is no good for someone who like to pinch the HDD space (Like me) because it mean I must allocate the additional HDD space at about three GB for the ISO file. I didn't play Kiminago yet and it also got the translation patch as well, although looking from the Flyable Heart which obviously share same engine as Kiminago I would expect same problem to occur.
For Flyable Heart, there's three set of routes that sort of serve as question-answer relation like back at Higurashi, and those three pairs are Mayuri-Sakurako, Amane-Kururi, and Yui-Suzuno. While there's still some plot element that I didn't find it quite satisfying (Mostly at Kukuri's route), overall Flyable Heart here manage to complete the main story. For Kiminago, I believe that it's for Mayuri's fans who didn't find her development in regard of family issue at Flyable Heart satisfying, although to me it make sense that they wrote Mayuri route in Flyable Heart like what it does there to a degree because Mayuri's family issue here is almost unrelated to the main story. Whether you'll play Kiminago or not I would say that it depend on whether you like Mayuri at the main game or not, and if you want to see more of her after reading her route then sure you can play Kiminago. So yeah I recommend you to read Flyable Heart first, and whether you'll continuing to Kiminago or not later it's up to you. Either way have fun.
Back to the topic, and as for hidden gem I would say that it's quite hard to say here. That said I can still give some VNs that I wish can be more popular such as Kichikuou Rance, Hatsukoi Sankaime, Kami no Rhapsody, Seabed, and Galaxy Angel trilogy. Oh and Flyable Heart here is also count.