Abysmal single-player. Multiplayer was the main attraction, but it's no longer alive.
The best part of Halo 2's single-player is the addition of the energy sword and the ability to dual-wield weapons, at the expense of losing your grenade-tossing arm. One- or two-shotting foes never ceases to entertain with the energy sword, while dual-blasting Brutes and Elites with double energy rifles scratches that action movie itch. The new weapons add a wrinkle to strategy where you must decide whether to go with the long- or medium-range weapons of the original game, or mix it up with short-range weaponry.
The worst part of Halo 2's single-player is, unfortunately, everything else. Starting with the graphics and art direction, Halo 2 hasn't aged as well as the original. I think it's mainly down to the first Halo's excellent use of light and shadow. Here in Halo 2, the environments are generally much brighter and the ugly textures are much more noticeable. Facial details are a little more refined, and human and alien creatures look a bit more realistic in cutscenes, but the level designs are a big step down from Halo 1. Repetitive, boring sci-fi levels fail to catch your attention as well as the original's iconic levels. Levels are also confusing to navigate. You'll frequently waste time trying to figure out where you're supposed to go next, or where you've already been. Progress is poorly signposted. It's often unclear if you even need to finish combat, or if respawns are infinite and you need to just run to the next checkpoint, wherever that may be.
The storyline is equally lackluster, putting heavy emphasis on the Covenant's internal power struggles and basically replaying the events of Halo 1. Fight on the surface of Halo! Stop the bad guys from activating Halo! It's really bizarre how Halo 2 is basically a retread of the first game's plot, but here they try to mix it up by letting you play as one of the Covenant in a muddled side-storyline that never captures your imagination. There's enough plot exposition where if you're a trilogy completionist, sure, it's worth it to play through it before moving on to Halo 3. Perhaps we're lucky, then, that the one saving grace is the campaign is over before you know it, abruptly ending after a minor boss battle and you'll be ready to move on without spending too much time with the game.
All in all, bleh.