I love the last boss, went up against him with the X-02 (which meant selling every other plain I bought to get it, but visually worth it). Hard does seem a bit more difficult at first, but once you get into the groove of it, I don't think it's much more difficult than 5. Only major difference in expert so far seems to be that you're taken out with a 1 missile kill, which is no different than Ace difficulty in the other games. Story is a bit weak, but thankfully the FMV isn't anywhere near as bad as 10 years ago. Plus Espada-2 is pretty hot. Overall, this one seems like it's geared towards air vs air fans, which is fine since 5 seemed to focus more on prepping you for the tunnel battle at the end for half the game.
Spoiler...
Pixy's plane does look suspiciously like the YF-19, while the X-02 looks suspiciously like the YF-21, playing chicken at the very end was suprisingly fun, though I can't wait until that plane with the laser unlocks, much prettier than 5's Faulken.