Summary
Simple platfrom action and great fun.
The Good
As the direct sequel to Commander Keen 1 : Marooned on Mars, there is very little difference. Sure the areas, puzzles, monsters, etc., are all different, but it is basically the same game.
Personally, I prefer this to the first episode. This time, you're aboard the alien's spacecraft and you must destroy all the death rays focused on Earth's major cities. The levels are kind of cute, and I definitely got a kick out of running around their spaceship.
The Bad
Since it was based on the same engine as the first episode, it suffered from the same problems. Particularly, the awful idea that CTRL+ALT should be the fire control.... stupid, stupid!
The Bottom Line
As a continuation in the series, it's a must for anyone who loves Commander Keen. The story is as strong as the first, and the levels a little more entertaining. A definite in my book.