The series itself has some good and bad points -- its good being very good and its bad being mediocre at best. All Warhammer 40k games are real times strategy, though there's a big difference between Dawn of War 1 (plus its expansions) and Dawn of War 2.
The first game is in many ways reminiscent of a 4X game. You had to conquer points of interest on the map to gain more resources, so you could then build more troops and overwhelm your enemy's base -- all the while preventing your opponent from doing the same to you. Meanwhile, the second in the series is much more focused on small unit tactics and the ability to create troops or buildings is almost completely gone.
Almost any game in the series is good. But you can probably skip over Soulstorm (the standalone expansion of Dawn of War 1), which is the worst of the series due to bad voice acting and rushed programming that made it buggy.