One of the most balanced FPS with an extensive ammount of content
Halo 3 is, in a word, excellent. Bungie's efforts are clear as every element of this game shines. The campaign is excellent, the multiplayer maps are excellent, the weapons are excellent, you get the idea. It is hard to criticize, but there are some elements that keep Halo 3 from being perfect, but it mostly from the community of players and not from the game itself.
The campaign mode is compelling. Despite some people's complaints it is actually quite long. On solo the difficulty scales really well. One of the greatest things about Halo 3 campaign is that in addition to solo it can be played co-op with up to 4 players (two on one console and the others online). With the extra people even legendary might seems a little easy, but that's fine because there is still more to do. There are secret skulls that can be collected in the campaign and turned on the make the game harder. In addition scoring can be turned on to turn the campaign in to a competitive cooperative multiplayer. Its fun and is worth experiencing more than once.
The multiplayer on Halo 3 is also exceptional. The map designs and the weapons are all so well balanced. The ranking system on the game is compelling and does a great job of matching you up with your skill level. In addition to ranked matches there are social matches that can be played without regard to player skill or rank. There are plenty of modes to be played and if the matchmaking playlist aren't enough, there are always custom games where you can set the options to whatever you prefer and play against friends.
In addition to the standard campaign and multiplayer there are forge modes and a theater mode. In forge you can modify the stages to you hearts content. You can't change the maps but you can modify everything about them including weapons and spawn points and any other object in the level. Also Forge can be done with more than one person, allowing for collaboration and speeding up any efforts (or you can just kill each other). Theater mode is a great addition because it allows you record those stellar moments when you kill an entire opposing team of pull of some ridiculous show. It also allows for picture taking, which if you search has almost become a form of art in itself.
With all the things that can be done in Halo 3, and the fact that it can all be done locally or online with your friends, and the fact that it is all of such high quality, it is really hard to not recommend Halo 3. If the game has any problem it is that you'll often get annoying 13 year old who ruing your online match, but that isn't any different that any other online game with a large following. Halo 3 is about as close to perfect as a shooter can get. If you have a Xbox 360 you owe it to yourself to pick up this game.