The story is interesting, coherent, and engaging. The characters have personalities and there's plenty of humor in the cut scenes and the levels. The levels are exceptionally well designed; each map is so unique and detailed that you really don't mind the older graphics (an issue that really affects other old games like Halo).
The gameplay is also well designed. I was more used to shooters so I passed the first few levels with guns blazing. But by the end, I was sniping enemies, sneaking past guards, and disabling cameras. It turns out that the stealth style suits me better since I have very good shooting accuracy. I ran into quite a few annoying bugs and the had to change the default settings a lot to get the game to respond how I wanted, but I still have a lot of fun playing the game.
Overall I enjoyed this game a great deal. The story, humor, and gameplay are so well thought out and produced that you won't be disappointed.