I've only played the first 11 missions. The storytelling via the computer database system e-mail is really cool. So far it is pretty easy. I also think the audio is your top priority to fix. I could imagine some really neat synth music would fit perfect with this project.
Maybe the difficulty picks up, and I'll be playing more of it later, but I think you should challenge the player earlier into the game.
That being said the transitions between levels and menus are flawless and overall it feels about how you'd want it to feel.