Surprisingly good fun!
The opening scene gives a tantalising and clever glimpse of what your character will be capable of much later in the game - you are immediately introduced to this side-scrolling platforming shooter, in which your protagonist collects loot, gadgets and power-ups to build up a power suit that enables you to navigate the map with increasing ease. This game was built using the Unreal Engine, which shows in its rich graphics and cut-scenes.
The stealth take-downs and 3rd person (in 3D) over-the-shoulder turret sections bring a nice variation to gameplay that is already interesting. There are varied ways to take out enemies, from stealth through blazing guns to manipulating your environment. The puzzle elements of working out how to get into certain rooms by collecting and exploiting new weapons and power-ups is compelling, and often frustrating.
For me it was reminiscent of Portal (especially the 2D version) in that there is just something elusively *cool* about the game that you can't exactly pin down. In some games you just don't care about the universe the developers attempt to create and you just blast through it from start to finish, whereas with others, you are drawn into it and feel like you have a vested interest in exploring the universe and seeing how the story turns out. This game is one of the latter type, you really want to finish it and see the conclusion - which in my opinion is one of the better endings to a video game that I've seen.
The only complaints I would have with the game would be that the first time I played it, the objectives system didn't work properly for me and guide me through the map correctly, meaning I had to restart the campaign - this wasn't such a terrible thing though as it is thoroughly enjoyable. The campaign is a litte short once you've worked out exactly what you're doing, but as I mentioned before, it's well worth replaying a couple of times.
Have Fun with Shadow Complex!