My advice for Final Fantasy XIII is to fight every single battle you can - don't be selective at all - before you get to the free-roaming/world map part, which is pretty far into the game. After that, it will be obvious that you have to be selective (giant, building-sized creatures march around with a little crew of normal sized scragglers, and you have to work your way up to those challenges). Don't get discouraged by the linearity and boring characters. XIII is all about the battle system, which might be hard to believe since it's easy to think, "oh - so I can't control anything, this sucks." Give it some time, and you'll realize that it is the most awesome battle system in a Final Fantasy so far. When you roll up on a boss that is absolutely impossible to beat, take about 20 minutes to re-group your party in the menu, change the battle strategy set-up, and re-enter the battle with your new roles. The key to being a badass in FF XIII is to constantly switch paradigms in battle, like you are popping a clutch in a Ferrari going around a track with a lot of twists and turns. It is the only Final Fantasy I've ever played where you can totally change the balance of a battle just by changing your strategy.
A lot of hardcore Final Fantasy fans will talk shit about XIII, and rightfully so, I guess, but it is actualy very hypnotizing when you are playing it - and when you catch your stride with the battle system you will see that it rarely gets boring. The locales are stunning, and despite the lack of typical Final Fantasy masterpiece music, the ambient music and sound effects are on par with the classics in their own right. My only complaint with the Xbox version in the choppy frame rate when you're in super-expansive locales with tons of species of creatures roaming the fields.
To be honest, I got pretty far and quit. I just lost interest, but will definitely go back once I get some time.