As others mentioned, he has quite a challenging start. But it isn't so bad. Given the load of fighters/bandits you fight in early game, he has WTA for a nice bit of early game to give him a nice boost start to keep up with the other Greil Mercenaries. With growth rates like that (Plus any band of your choice), RNG screwage is close to rare. Though he'll most likely still have problems dealing damage and taking it if he refuses to gain any strength/defense on the first few level ups.
Mid-game, he still has his uses. Since this is HM, he won't jump to level 20 until around say...Chapter 14-16 ish if you are constantly putting him to work and trying to level everyone up equally to at least have a few promoted before he does. Given that he promotes just about 2-3 chapters after that, you basically won't have to wait too long to start placing him back in the front lines.
From mid to late, he destroys the game. Literally. By promotion and on Chapter 18-19, he's already solid enough to be late game material after a couple of level ups. He receives some pretty sweet promotional gains (Mainly +3 gains in certain stats). He then breaks the game if you feed him an occult. Though against the Black Knight and Ashnard, it's a little unreliable that Aether is your only hope to dish out some good damage for the kill if you don't want to waste skills on Ike for those battles only, since those skills can go to better candidates who need them more desperately than he does. But that's only if you want Nasir (Not factoring him in my rating). :\
I have no hesitations on giving my favorite character a 10/10. Not because of bias, but he's most likely one of the best Beorc units to have on your team. Plus he's forced. Takes little effort to make him tough as a rock.
Edited by Magnus Blade, 27 March 2012 - 04:28 PM.