Basically, if you liked Halo, you will like its sequel. Here's a rundown of the new pros and cons:

Pro: You can now use some weapons two at a time, allowing for more raw firepower.
Con: The shield/health system that worked well in Halo has been scrapped for a harder-to-interpret, seemingly easier to defeat system.
Pro: Graphics are better.
Con: The ending is a total gyp (no spoiler here, but if you really want closure you'll have to buy a 360 and play Halo 3).
Pro: More weapons, and improvements on existing ones (the needler especially).
Con: I found a few rare, intermittent glitches, mostly with the Arbiter levels.
Pro: Multiplayer is vastly improved, now with XBox live.
Con: While the levels are more varied, they don't stand out as much as some of the memorable levels from its predecessor.
Pro: The plot is much deeper.

So, all in all, it's more of the same, which isn't necessarily a bad thing. There are some minor improvements, but there are a few things that could have been better. A good buy.

having never played h3 but more than a couple times, i believe this is the best game in the world.

and if it isn't, its only second to halo three!

Liked the game, however...had to return because of shadowy images in several areas which covered up part of the picture. Returned game and had replacement issued. Had same problem. Returned again and received refund.

Halo 2 is perhaps one of the best games in history! At first I played it at a friend's house and then I decided to buy it. The Graphics are very realistic and it feels very intense. Plus you can play as Arbiter, an Elite! Your enemies such as, Grunts, Elites, Jackles, and Hunters can also be your allies! Halo 2 is just really fun! I can't wait til' I get Halo 3!