Crisis Core: Final Fantasy VII is a game that should surprise everybody whoever plays it for all the right reasons. It's terrific fan service to those who love Final Fantasy VII and the series in general. While the game is quite linear, it actually works out as one if its strengths because it makes it a lot more approachable to players who don't consider themselves hardcore RPG players because of their extraordinary lengths.
The main story should take close to 12 hours to complete, but the side missions could take you well over 30 hours, maybe even more if you want to beat the game the perfect way. Luckily for those who blaze through, you can play through the game again in New Game + to import all your original stats, and it makes the game have that much more replay value. If you have the slightest interest at all, Crisis Core: Final Fantasy VII is worth your time and money.