MULTIPLAYER IS THE BOMB!!!!!!!
I've spent over 100 hours playing MULTIPLAYER and literally zero playing the campaign and skirmishes. I understand that there are different expansions in this Dawn of War series like Dark Crusade and that Soulstorm only adds two new races to the game but... since I have all the races unlocked in multiplayer it doesn't make a difference to me.
THE MULTIPLAYER IS AWESOME. If you love RTS multiplayer, then you will love soulstorm because the game is simply amazing when you play against other people. If you have all the other games, then you'll have a ton of races to choose from, each with very very unique characteristics. I can't describe how amazing the game mechanics are, the game is incredibly balanced and having played every single race, they are are equally amazing in their own respects. Also, there is very rarely lag in multiplayer games which I find very surprising given that you are playing against people around the world, literally...
I've played against french players, russian, czech, korean, chinese players all on Soulstorm and it's so easy to get a game started and finished. Long games run up to 30 minutes while typical ones are under 20 minutes. This may sound fast but it lets you get hundreds of games in your repetoire such that you don't suck anymore. Once you're not a noob anymore, you will find playing against other people incredibly fulfilling. The game is so diverse and dynamic with very unique attributes and abilities for each race such that you can easily play over 100 hours in multiplayer without getting bored.
This is the game that has made me drool over dawn of war II because once you've played soulstorm multiplayer through and through you will see how amazing DOW II is going to be.
If you're only looking for a campaign experience, you'll probably get bored within a few games because the experience is 10 times more dim than multiplayer against real human opponents. AI opponents can be "turtled" out, where you sit around for 30 minutes building an unstoppable force to crush them. This gets so damn boring too fast, but with multiplayer, you're always on your feet!
I hope this has helped you make your decision!!!!!!!!!!!