Halo 2 was a bit of a dissapointment.
After all the development the game still carried Halo's flaws with only a few features worth mentioning. The game still features repetitive level design and battle sequences, but it's the plot that was the real let down.
In short the story is boring and predictable, and if you have played the first Halo you have heard it all before.
The only new features are upgraded graphics (which have shocking pop-up in the cut scenes), the ability to dual-wield weapons, and also the ability to board enemy vehicles ala GTA.
Maybe worth a rental before you rush out and buy it, it's really “Halo 1.5”.