Yeah, that takes a lot of work, but it's really rewarding too. I've been in choir since birth--(okay, since kindergarten)--and I've gotten to sing selections from the Messiah a few times. A year or so ago, actually, we did the whole Christmas half, and we ended with that chorus (which comes later in the piece) because we knew people wouldn't be satisfied without it. What's cool is that people are always encouraged to stand during that movement, so it was a really easy way to get a standing ovation for the concert!
One of the other choruses from the Messiah that I love singing (especially at Christmas) is "For Unto Us A Child is Born"--it really fun, once you get all of the runs down...