He truly has a knack for turning songs you hate into songs you love. It's amazing what a solid lounge act with a great sense of humor can do for previously awful music.
That's not to say they don't cover good music as well... quite the contrary. Their cover of "Hot For Teacher" is fantastic and really shows of the piano player's skills. But still, what I find most impressive is how they can make music from bands like System Of A Down actually entertaining.