Playing Tap Tap Revenge and other similar games might teach you to quickly press buttons in time with the music you're listening to, but that's about it. Lyric Legend actually drills the lyrics of the song into your brain.
The idea is the same: A song plays, and you hit the screen in time with the music. Lyric Legend, however, shows you bubbles with the lyrics in it, forcing you to hit the right word, in the right order (it doesn't care which "the" you hit if there are two on screen).
It's a lot more complicated than it looks, because you have to engage both the part of your brain with music and rhythm detection as well as the part of your brain that recognizes words, then process the word and figure out which one to hit. Imagine playing Simon says, but in a language you only took two semesters of in college. There's an added layer of complexity.
The only complaint I have is that you're forced to buy individual tracks from Lyric Legend instead of being able to use your own music from your own iTune library. This is extra annoying because TuneWiki, the people who made this software, also makes TuneWiki, an app which fetches lyrics for all your songs and displays them in time with your music. I understand that there are licensing issues and the record labels want you to pay for music again, even if you've bought it once, but it is annoying. [iTunes]