I experience with RFG went like this:
First hour: bloody dull. The destruction is very cool but makes the game feel like a gimmick with a shallow game pasted on top of it. Missions so far seem incredibly shallow ("go here, shoot that").
Second through fourth hours: getting much more used to the game, liberated a few sectors now, missions have gotten mildly more interesting. And, oddly, the destruction gimmick hasn't gotten old, it's instead gotten more awesome. It's an absolute blast beginning your assault on an EDF stronghold by finding the biggest truck you can and plowing through the middle of it, then getting out of your truck and throwing shit-loads of remote detonating bombs all around. Absolutely glorious chaos.
Last two or so hours: great game! Story is meh. *Side* missions *do* get repetitive, but the story missions are marginally more interesting, if only for the story elements (but again, don't expect too much from the story). The last mission is awesome, though, as is the ending cutscene.
Great game. Multiplayer is a blast too. I'd pick it up!
Also: don't be afraid to turn the difficulty to casual. The game has massive difficulty spikes, and you can't believe how fun it is to cause epic chaos with a nearly invincible character (he's practically immortal on casual, and dies in an instant on medium).
