I have Birth by Sleep and I love it. Because it explains the most important thing which was necessary to be explained: Xehanort's story. Why do we see a youn Xehanort in a Kingdom Hearts II flashback while the Birth by Sleep Xehanort is an old keyblade master? Everyone who knows Unlimited SaGa and has finished Judy's story (with beating the antagonist of her story part - Clyde Blackstorm) can think what happens - but doesn't know how it happens. Plus, the secret ending explains very much and proclaims the sequel of Kingdom Hearts II - and I hope it is the final part because it is enough of the main story. Xehanort shall return fight Sora, Riku and Kairi, Happy End, finished. That's what I want.

To Birth by Sleep itself - I really love it. As much as every other part of the story.