It may not be the definitive version of Halo 2, but it's a great reboot of an Xbox classic that holds up well today.
In a brief summary, if you played Halo 2 before on your Xbox/360, if you beat the first Halo for PC and were interested in continuing the plot, or you have somehow never played a Halo game before, you'll be glad to know that after nearly 4 years (7 including the time after the old Xbox release) since it's release, Halo 2 is still a solid, balanced, and all around entertaining FPS. The game controls very well on a computer, and even if you've played Halo for Xbox since it first came out, you shouldn't have much trouble adjusting to a mouse and keyboard. This game is actually one of the first shooters I've ever played on a PC, and after only a couple hours I was ready to compete with the rest of the world online. And even if you can't stand the PC control layout, you can simply plug in a 360 controller and play it like you used to, which is a very nice touch.
The campaign is just as fun as it ever was, and it remains one of my favorite Halo campaigns to date, since its the only one with 2 interlocking storylines, boss battles, and other little inventive touches. Given that the third game is out, the big cliffhanger wasn't annoying to sit through in my opinion, but if you're one of ten people on the planet that hasn't played the third game, then you may be frustrated with the ending, even more so since Halo 3 isn't slated to release on PC. Otherwise, it's a fun campaign that'll get you used to the basic gameplay and give you an immersive, sci-fi storyline to go with it.
The multiplayer is where it's at, and thankfully it's just as fun to play today as it was back in 2004. The game has been perfectly balanced, has a multitude of options for customization, and on Halo 2 Vista, comes with 2 exclusive levels (which fit in surprisingly well with the other maps and are actually above average. Uplift is one of my new personal favorites!), achievements (for campaign and online), the ability to join lobbies along with matchmaking, and a complete map maker to let gamers craft their own stages for other players to test out. It's a little difficult to get used to, but it IS able to create pretty much whatever you want. As of now I've seen some sick original stages, re-imagined versions of older multiplayer battlegrounds (Summerwinder anyone?), and remakes of levels from Halo 3 and Halo Reach, which are actually quite admirable. It's a very powerful tool, but it just takes some getting used to. Either way, you don't need to make levels to enjoy the ones that other users make, so it's not a huge deal if you avoid it.
There are a few issues with the game however. For one, the game's graphics are pretty weak (considering how good some other PC games of its time looked), and on my laptop from 2009, I have to run it on minimum setting for EVERYTHING to get it to run without stuttering and lag. Requirements that high are inexcusable for a game this old, and its more than a little absurd that I can't just use the higher settings. So unless you have a PC capable of running it, you're stuck with a game that looks much worse than the Xbox version, which released 3 years before it. Dumb.
Secondly, although this isn't really the game's fault, the community (especially considering that it's a Halo game) is relatively small. You'll never really have trouble getting into a full game, and the community has begun to grow slightly since Halo 2 Xbox's death (RIP btw), but it still doesn't get much more than 100 people, and they usually stick to the same 7-8 maps and Fiesta, CTF, Swords, and Shotty Snipers. Occasionally you'll find a lobby with people that deviates from that norm, but that's a pretty big "if". This in turns means that you won't be able to play some of the game modes you loved in Halo 2 before (still haven't played any Assault or King of the Hill games :( ), and can occasionally limited to the most popular maps from Halo 2 Xbox.
And lastly, the game has NO ranking system or stat tracking of any kind, which is unfortunate considering Bungie.net's robust stat tracking on Halo 3, ODST, and Reach, and even Halo 2 Xbox. On one hand, it makes the game much more social, meaning you get less whiny, complaining gamers who want their K/D ratio to stay high. But you lose the addictiveness of trying to up your ranks, which can limit the long term appeal of the game. You also lose the co-op mode from the original release, and the framerate tends to stutter more than before, which is unfortunate as well.
In the end though, Halo 2 for Vista is a pretty solid port of an Xbox classic that mostly preserves what made the first release so great. It's a shame that the game didn't get much of a graphics boost, and it's surprising to see so many features cut from before, but the gameplay is just as great as it was before, and the added Map Maker, achievements, Games For Windows Live support, 2 new maps, and the fact that it's the only version of Halo 2 with functional online to work in it's favor. Overall, a solid pick for any Sci-Fi shooter fan! :D
[Where Should You Buy It? Platform Comparison]
+ = PC
- = Xbox
-If you have never played the game before on Xbox and don't own a console, definitely get it on PC to continue the story and relish the excellent multi-player modes. (+)
-If you own(ed) the game already for Xbox, whether or not you should buy it is based entirely on how much you love achievements and want to play Halo 2 online over Halo 3 or Reach. I bought it because I missed out on Halo 2 online, and it's still a fun, worthwhile experience. (+/-)
-If you don't have internet (somehow), it's an easy one. Pick it up on Xbox instead for the co-op mode and extensive split-screen support. (-)
Personally, I would recommend the Xbox one over the two because of the co-op mode (which is how I prefer to play Halo campaigns), split-screen multiplayer (which I play quite a bit due to living in a college dorm and having plenty of friends around who are willing to play), and, at least in my case, for the better graphics and framerate.
However, choosing the Vista version isn't exactly a bad choice. You get the online missing from the Xbox version, achievements, GFWLive support (so you can talk to friends that are playing Xbox. Sick.), and a couple new features to compensate for the things that were removed. It's pretty much whatever feature set you would rather have, I just happen to make better use of ones on the Xbox.
Hope my review helped you make a decision! :)