Its fans think it's fantastic, one of the best RPGs ever. It was notable for being very long (70+ hours to complete) and having a very complicated, involving story. Unfortunately, its story was really too long to be completely told in one game, and so towards the end lots of things are explained in text screens rather than being shown as story. It also suffered from excessively frequent random battles.
Xenogears is a game about a young man who has no memories of his past who is suddenly thrown into a conflict that will decide the fate of the entire universe. The story is very detailed, with references to a history that spans thousands of years.
The battle system is based on combinations that are executed using the buttons on the Playstation's controller; it was probably the model for the control in another Square RPG, called Chrono Cross.
Xenogears' story has many religious references and (rumor has it) was almost not brought to the U.S. because of them.
Square declined to ever develop a sequel/prequel. However, some of the original developers have left and created a company called Monolithsoft, where they are developing a prequel called Xenosaga: Episode I The Will to Power. This is being released by Namco in Japan in Feburary 2002 for the Playstation 2. Eventually, they hope to bring out enough games so that Xenogears will be part five in a six part series of games.