To me the main problem with the Compilation of Final Fantasy VII so far is that it's been too far detached from the lore of Final Fantasy VII. Before Crisis features little of the characters from the game, Dirge of Cerberus focuses on a new villain and secret character Vincent and Advent Children is simply eye-candy.
Enter Crisis Core - a game set chronologically before the events of FF7 but overlapping with several key moments that appear in the game via flashbacks. The events of Crisis Core are arguably the most important events in the FF7 universe, so those hungry for a relevant in-universe expansion are in luck.
Crisis Core tells the story of Zack who at first glance might appear to be a black-haired Cloud lookalike. He's the man who Cloud looks up to and eventually bases himself off when he loses his memory, so he'll be instantly familiar.
Even that isn't all bad - towards the end of the game the DMW is used to incredible effect to aid the storyline along, and it helps the tragic ending become even more effecting.
For fans of FF7 this is just one of many loving touches that will make the plot of the game moving, exciting and interesting despite the fact we all know how it ends. Areas of the plot are flawed and some of the new characters - Genesis in particular - may make you shake your head, but generally it's all good.
Crisis Core might even be a good option for non-FF7 fans and has been set up so it'll serve as a perfect entry point for those wanting to get into the FF7 universe. It doesn't expect anything of you in terms of knowledge and tells you everything you need to know.
Those people will merely see Crisis Core as a good game - but FF7 fans will see it as so much more, a game designed to play on every fanboy-like urge in your body that playing a classic like FF7 instills. Zack is a fantastic leading man and the FF7 universe is presented with such style and clarity that it's arguably better presented in some places than the original game.
There are flaws, but combine addictive gameplay with pretty graphics, a moving plot and a clear feeling that this game was treated with as much love and care as any main-line Final Fantasy, and you've got your first classic FF7 spinoff.