Of course a big fast serve is helpful, as it can win a player many free points and make life easier on the court. But if you look at the 4 players that reached the semis of the U.S. Open, they are not huge guys. They have good serves, especially Federer and Djokovic, but they don't serve bombs. They make a high percentage of their first serves, they have a reliable second serve and their placement is perfect.

To sum it up, I would say that, yes, it's great to serve rockets over 120 miles per hour, but it is more important to place the the ball in the right spot and to mix up the pace of the serve from time to time (like a baseball pitcher). Sometimes a 90-mile-per-hour serve, if it's well placed, can be just as effective as a huge flat serve. Some players actually prefer to return a faster, more flat serve.