Besides the already stated horrible lip-syncing and touch-ups on their voices, I think this was a great episode! The music wasn't that good, but it came together SO well. I didn't understand the name of it much until the ending conversation between Emma and Will. "They don't belong together, but they're both good songs." It defines so many of the pairings in the show, which is why it worked out so well.
And, I have to say, Rachel is SO cute in a miniskirt. Also, Sue is freaking amazing.
So yeah, while the music was kinda lousy in this one, the episode itself was great.