It's a good game.
I won't say Halo 3 is perfect, but it is a good game. The single player campaign is a little short (8-12 hours), but it's fun and entertaining; it becomes ten times better if you're playing co-op with three friends. As you would expect from a Halo, the Multiplayer is a lot of fun. There's a lot of customization with both the game modes and maps (thanks to the forge map editor), which makes custom games a blast with friends. Match Making is also a lot of fun if you don't mind playing with people you don't know; there are alot of playlist, which means there is likely one that fits you style of play (my favorite is Team Objection). Add the awsome replay function (which allows you to take clips and screenshots from multiplayer matches), and you have yourself a very nice package that has plenty of features and a lot of fun. Halo 3's presentation is top notch, and the sound (from the sound effecets, to the voice acting, to the amazing music) is some of the best in the biz. While the graphics aren't the best I've ever seen, they are fairly nice and do get the job done; what Halo 3's graphics lack technologically is more than made up by it's awsome art style and color scheme (yay, I see more than just brown mush). The gameplay and controls feels a lot like Halo CE and Halo 2, which is a very good thing; Halo 3 has some of the best FPS controls on the Xbox 360 (or any other console for that matter). Halo 3, while not perfect, is a very good game, and well deserving of your $60!
Note: I am a big fan of the Halo series, so you should probably take anything I say with a pinch of salt.