Although Faith in Destiny, the previous (also chronologically) installment of Spellforce series, was slightly disappointing, Demons of the Past is an amazing game, especially for the fans of the Spellforce series.
Gameplay - RTS and RPG mix still exists. Although I always liked Spellforce 1 (platinum edition on Steam)'s RPG system (skill building, etc.) more, the RPG side of Demons of the Past is pretty great still. Many side quests and main quests give you many details about the story, making it long, good and rich. RTS, on the other hand, is really good - challenging, yet easy to do still. The system also makes it easier to use some of the skills, spells and other special abilities. Army often can even do it on their own. It may be a bit hard for the newcomers at the start, but not too hard.
Soundtrack - as always, soundtrack to this new part of the series is pretty good.
Voice acting - it has certainly improved from previous titles.
Bugs - there may be a couple, but nothing too big. They haven't really bothered me.
I haven't played multiplayer with DotP much yet, but previous titles were already a lot of fun. You can even play the campaign with someone cooperatively.
Overall - 4 out of 5. It brings a good end to Spellforce 2, similar to Dragon Storm, and perhaps equally dark to Shadow of the Phoenix. Although the engine is old, it was improved. It may have some bugs and/or a downside or two here and there, but it has been a very enjoyable game with a very good story and also a good to Spellforce 2.