While Iron Sky: Invasion (and its add-ons) is not the end-all be-all of space fighter simulators, it probably doesn't have any right to be as good as it actually is, especially for a video game based on a movie. The whole of the game took a fair portion of the latter half of the film - basically the entire space fight and Earth fleet reinforcement segment - and stretched it out without making it boring. The tone of black comedy and "just run with it" silliness from the movie is preserved, from the mission briefings with Madame President Totally-Not-Sarah-Palin, to the political shenanigans of the various national representatives, to the over-the-top voice acting from the various pilots in the fleet.
The controls are keyboard-and-mouse, primarily mouse oriented. On screen contextual tips do show up from time to time, which can help if you go a long time between having to use a specific feature of the ship. The learning curve for controlling the ships is very short. There are some technical problems - while I haven't noticed any crashes or freezes, it doesn't like my graphics card and the mission briefing videos lag the entire game as often as not. They can be skipped with no penalty and watched later, it's just distracting.
Unfortunately, once the tutorial is over, the difficulty curve spikes pretty hard; it's all but impossible for a new player to know what to do and how to do it. There's a certain amount of flexibility - if a squadron of Nazi ships makes it to earth it just applies a penalty to reputation, which is a score by any other name - but there's no option to turn down or postpone side missions, they get added to your to-do list automatically when you head to a friendly station for resupply and repairs. When you're running all over near earth orbit trying to play space tag with the Fourth Reich, something's gotta give. That said, you can buy yourself time to fight the smaller fighters and do side missions by taking out the engines in the big ships, leaving them adrift - but the only mention of this strategy comes up once during the tutorial on sub-system targets. It's very easy to miss.
Having said all that, if you liked the movie, you'll probably like the game. And even if you didn't like the movie, you might still like the game. After all, it's a game that lets you fight Space Nazis. I'll repeat that: You get to fight Nazis. In Space. And who can honestly say they never wanted to do that?