Lets face it: the Half Life 2 vs. Halo 2 debate has gone on for quite some time. Being as I own both, and have a lot of opinions on both, I've decided to give my opinion on the debate.
First off, lets look at Halo 2. Halo 2 was created for the XBOX by Bungie, basically a sequel to the original Halo. Instead of going outside the storyline box, they rehashed the old story with a revisit to a "New" Halo. Of course, this begs the question as to why the first Halo from Halo 1 was such a big deal if there were 9 Billion of them, but lets ignore plot holes for a second here. The story is famous for having the worst ending... oh wait, not even a god d*** ending. Apparently, the people at Bungie decided to completely stiff you of a ending, leaving you with a shameless reference to maybe a Halo 3. Great, but it reminds me more of the ending to The Matrix Reloaded than anything else.
As for the good points of Halo 2, its just like the old Halo, so it is very easy to play... and win. The game is simplistic, throws legions of enemies at you, and generically has you blasting through volleys of enemies in an attempt to get through the level. The game did try, as hard as it could, to break the monotony of the first Halo, but there wasn't much new. I beat Halo 2 in a few hours without breaking a sweat, you should too.
Halo 2's big feature is the multiplayer. Available in splitscreen multiplayer and XBOX Live, the big idea is to take the original Halo and bring it to the internet in an official manner. This would be great, but there's one issue: There isnt much to do new. When you connect online, all you are doing is playing the same maps with the same weapons with the same modes, regardless of where you are or what you are doing. In any given battle, the best weapons tend to be either the rocket launcher, the shotgun, or dual SMGs- don't expect any kind of weapon balance in this game.
Half Life 2 took about 6 years of development for Valve to actually get this freaking game out. But there's a sign that this game was going to be good: The second the screenshots came out, Video Cards sold like hotcakes. That's because people who play shooter games knew this was going to be good, and they were'nt dissapointed (Other than that extra 2 years after they got the new video card).