I think Kat's right that the cover of "Empire State of Mind"—to be featured in tonight's season premiere—doesn't quite work. While I generally like the show's covers more than I've come to like the show itself, I think it makes a couple of mistakes with material.
First, it sounds incredibly simplistic to restate that rapping and singing are distinct skill sets, but Glee seems to need the reminder. Matthew Morrison, for example, is much better covering "Alone" than he is throwing down even that white-boyest of rap releases, "Ice, Ice Baby." The kind of over-articulation you have to do to make sure the back row in a musical theater performance gets all the lyrics in a show tune makes rhyme-delivery sound ungainly and unnatural. Everyone who has rapped on Glee is guilty of this, though Kevin McHale perhaps a little less so, both because "Can't Touch This" is slower, and he did it with a sense of irony fully intact.
Second, the songs work better when there's some tangential connection between the characters and the material. The "Empire State of Mind" cover appears to grow out of a plea by the club's coach to "show them how down we are." But they're not "down." They're a bunch of whitebread teenagers from Lima, Ohio. The kids who experience financial hardship deal with it by hoping for football scholarships or holding bake sales, not returning to the "corners where we selling rocks." "Total Eclipse of the Heart" is not, objectively, a better song than "Empire State of Mind." But the show's cover of it is substantially better than the "Empire State of Mind" rendition both because the people singing it have the relevant chops to elevate it, and "Total Eclipse of the Heart" fits the emotional moment of the show.
I'll be curious to see if the overall quality of the tunes declines if the club reaches further outside its emotional and musical context.