Yes nearly 100 years young and the theory is as rock solid as ever. (Newton's theory of gravitation remained unchallenged for over 300 years.) It is consistent with all observation. The latest test of one of the theory's predictions is being carried out by the gravity probe B which will measure gravitational 'drag' due to an object moving through the curved spacetime generated by another object (the Earth in this case).
The trouble with general relativity is that it does not play friendly with quantum mechanics, the other triumph of 20th century physics. What this means is that we do not understand how gravitation works on very small scales and, more importantly, we do not understand the fundamental nature of physical reality as a whole, in a consistent manner.
Check out the wikipedia pages (and links therein) on gravity and general relativity:
http://en.wikipedia.org/wiki/Gravity
http://en.wikipedia.org/wiki/General_relativity