Although not as good as rumored to be still an amazing game.
The combat in the first Halo was so finaly tuned that it managed to break the ice that had so longed the fps on the consoles by implementing some of the best controls, and in Halo 2 the controles are just as responsive and feel just as perfect and is one of the few fps's that I can stand to play on the consoles. Halo 2 did add some new elements to the game that changed the game dramaticaly. The first thing was dual wielding. This allows you to use two weapons at once firing with the left with the left trigger and the right with the right trigger. Another edition that I do agree with is the commandeering of vehicles. In the last Halo, vehicles could dominate a match, and unless you were equipped with the proper tools you did not stand a chance. Boarding allows you to kick the driver out and steal his vehicle. This prevents players from sitting in a tank and spawncamping everyone. One of the biggest additions was the support of xbox live. The way bungie set this up is the best online expierence on the consoles. It gives you a level based on your skill and will try to find other players within the same skill level. This allows you to get into matches were you will have a good chance of having some fun and not getting wasted. Also in the original Halo there was a problem with level design as many of the rooms were the same with just a few different enemies. In Halo 2 this problem has been fixed. Although once in a while you will see a repeating room or too.
There are a few problems with the game though. Dual wielding will help you with many fights in the single player but it can create some balance issues in the multiplayer side. The trade off was supposed to be that those with only one weapon can throw grenades. The problem is that the grenades do very little damage, and even after a patch to increase the damage they still feel a little weak. Despite receiving better level design the single player feels lacking, and the story feels much weaker and features a cliff hanger ending that may disappoint many fans of the series.
Overall Halo 2 is a great game. But, a few things kept it from becoming a true successor to the original.