Expands on the Warhammer 40k strategy games and improves them in almost every way.
The Good:
Lengthy campaign
Fast paced, exciting battles
Beautiful graphics and fantastic sound
Hero Leveling system is easy but effective and fun
Interesting story that ties well into the Warhammer universe
Smart and challenging AI, especially on higher difficulty settings
Most missions are fun
Campaign can be played cooperatively
Skirmish mode is great for quick action
Multiplayer skirmish mode can be fun too
The Last Stand is an absolute blast (make sure you patch the game as it's a free add-on that was included in patch 1.8.0)
The Bad:
Missions can get repetitive after a while
A few of the boss battles are frustratingly tedious
Can be difficult to find a multiplayer match with someone who's no a d%&#
I'll start off by saying this: Don't expect Dawn of War 2 to feel like Dawn of War. The series has ditched the base and army building parts in favor of more fast paced squad command. You command up to four heroes at a time, each of which has their own squad of between 2-3 additional units (with the exception of your Commander and Davian Thule). If you like building large bases and massive armies, you won't be able to do that here, but the squads you command give you more than enough firepower to conquer each mission. This also means there is no tedious resource gathering, which is a nice change of pace from the original Dawn of War games (especially considering there were 4 of them!). As a result the game plays like a hybrid RTS/RPG, similar to Warcraft 3...your squads level and gain new skills and items as they progress.
Gameplay is face-paced and explosive. You hop between 3 planets completing objectives on each. Missions often feel similar to eachother but are at the same time varied enough to keep things from feeling too repetitive, with the exception of defense missions. Defense missions are optional, however, and do not have to be completed, although doing so helps you later in the campaign, and they generally only take 7-10 minutes. You get rated based on how many enemies you kill, how many heroes stay conscious, and how quickly you finish the missions. Your score determines perks you receive, such as mission deployments and the number of times you can use special weapons, which can help a lot in later missions. If a hero is lost in battle don't worry, they don't actually die and can be revived by either using a stimulus kit or manually reviving them with another hero, so if you play intelligently and don't let yourself get overrun, you should never get to the end of a level with only one hero (it will still affect your score negatively if a hero is downed and revived). The difficulty can be adjusted to suit your style of play, and does a good job of keeping the game easy for new comers and causal players, and challenging for veterans of the series or hardcore strategy gamers.
The basis of the story is the alien Tyranid race is taking over and feasting on the Imperial planets. Without giving too much away, it's up to the Space Marines to stop the Tyranids, save the planets, and take find out what the Orks and Eldar are doing in the midst of the Tyranid invasion.
With the click of a button, you can also play the campaign cooperatively, although admittedly I have never done so so I cannot comment on how well the feature is implemented. You can invite someone mid campaign and they can drop out at any time, and it will not negatively affect your game progress. Again, since I haven't actually played cooperatively I'm not sure how gear is split up between the players, but I believe gear is simply shared among players. In addition to the campaign, you can play skirmish battles in single player or multiplayer. These are essentially quick battles that you setup and complete objectives that you can select from. Multiplayer skirmishes can be fun, but it is sometimes hard to find someone who's fun to play against...the community is your typical multiplayer community, so you will run into a lot of people who spam the same attacks over and over, poor losers and cocky winners, and sometimes you'll be in a match for half and hour only to have them drop out mid match. You can also play the newest game mode, The Last Stand. This can be played alone or cooperatively. In The Last Stand you take control of a faction leader and fight wave after wave of enemy units.
The game does use both Steamworks and Games for Windows Live clients. This doesn't bother me in the least, but if you have heavy qualms with either client you may consider that before purchasing Dawn of War 2.
Dawn of War 2 is an excellent game. There is very little that I could find wrong with the game...you really have to nitpick to find flaws. It's a great looking game, runs flawlessly on systems that meet even the minimum requirements (though if you only meet the minimum you will have to sacrifice some eye candy) and has no major crash issues / bugs. The two expansions that have since been released are equally fantastic, and I will be posting reviews for those shortly.