To be fair, TPTB haven't been consistent with any of the characters!
Regarding DC, I honestly think he consistently gives 100% to his performance, and does his genuine best with the material he is given. As with many actors, I do think he tends to up his game considerably when he's with either a better actor, or someone he has genuine chemistry with. (Personally, I'm not sure where they're going with the Klaine/Blam storylines, but I think a change of pace would definitely be good.)
Is Darren the best actor on the show? Of course not. But he's not terrible, and in my book, his earnestness makes up for that (at least in part), and we've been saddled with far worse actors over the 5 season of this show.
That said, Darren's biggest asset is definitely his charisma. Having seen him perform, I can safely say he veritably oozes charisma, and the small screen doesn't really do justice to him. Also, he seems exceedingly humble; I've yet to hear him say a bad word about any of his co-stars, or the atrocious writing of this show.
And of course, I still adore his voice; I don't normally buy much of the Glee music, but if his name's attached to it, I'm exponentially more likely to.