I think it's a fantastic game and a great step forward (and perhaps a step backward as well) for the series. Sunk over 120 hours into the game and have been going back to all my old FEs because it really whet my appetite for more. The story, though, was a pretty big letdown. I enjoyed the premise, but the plot devices were garbage. IS more than makes up for it in other ways.
And though I had a blast building up my characters, I'm not necessarily partial to that sort of approach to an FE game. The grinding seems to have started with Sacred Stones, and though it's still fun to have that opportunity, it can suck a lot of the challenge out of the game. I played Awakening on Hard/Classic, and it was challenging until about Chapter 4. Then, it opened up in a way that let me train my units into demigods. Still fun, but not really FE.
And yeah, Lon'qu's pretty bad ass. He's a crit monster. He's also got a great voice actor doing his blurbs. Definitely keep him alive; it'll pay off in spades.
I'm currently about 3/4 through my third playthrough, but I'm taking a break until some more DLC is released.