Final Fantasy XIII-2 is by no means perfect, and I am only giving the game 5 stars when comparing to the original.
The biggest flaw, and largest complaint by most players is that the ending is shallow and doesn't truly end anything -- true, but I actually am happy that Square Enix is leaving it open and continuing the game via downloadable content. This gives me a reason to hold onto the game and look forward to playing it again in the future.
First and foremost, for using a dated graphics engine, the game is gorgeous. It truly pops on an HD TV unlike many other games out there. The use of colors, weather filters, and combat details are excellent and unmatched.
Gameplay is far superior to XIII. Though it gets old watching the same two characters throughout the game, the monster catching system is fabulous! You can spend many of hours alone just catching and leveling up monsters to find your perfect companions.
This is also one of the few games where I actually LOVE grinding. The battle system is fast paced, dynamic, and the addition of context sensitive battle scenarios along with monsters having their own unique "Feral Link" abilities adds to the experience greatly!
It's great going to old and new locations freely, and being able to play the game as you see fit. The story is a little lacking, and it's more lighthearted (similar to Final Fantasy X-2), but it's engaging enough to keep you wanting more.
The music is also a big improvement over the original -- yes, there are a LOT of vocals on this soundtrack, but it feels more like a Final Fantasy game with the musical selections they've created. Not to mention the addition of Mog, and the strong focus on Final Fantasy critters and lore, which is something that Final Fantasy XIII lacked greatly!
Overall, I recommend playing the game -- yes the ending is irritating, but the game isn't over. Knowing that this game could literally continue for years is what's helping with the 5 star rating, and due to it's overall beauty and feel, you'll be welcoming it to your console until you get 100% completion!