leJOS is arguably the most powerful programming language for the RCX. It is a full-featured Java environment that allows integers, floating-point numbers, and just about everything that the official Sun Java platform has. leJOS stands for Java Operating System. It contains a JVM and also an API with many classes that help with robotics programming.
leJOS is so powerful that Simon Ritter, a Java evangelist for Sun, has used it (along with his RCX robots) to tour universities and tout the power of Java. For Ritter, the RCX brick is an ideal, portable way to give a visible demonstration of the power of Java.
leJOS was also recently used to power a robot that was sent to the International Space Station. The robot, which is called Jitter, was made for a zero-G environment. It bounces off walls, reorients itself using a gyroscope, and sucks up particles floating around the space station. The Russian cosmonauts who played with Jitter were impressed by the handling of x,y,z rotation, which they found similar to how the ISS itself controls orientation.
Why has there been so much enthusiasm for Lego Mindstorms? Probably because the RCX brick is a link between computers and the physical world. Usually, programmers can just sit and watch the results of their program by viewing it through the portal of their monitor. With Mindstorms, the programmers code is brought out into the real world, where it can interact with people.