Challenging
After hearing about how hard this game is, I just had to try it myself. And wow, this game is intense. I did complete it in the end though (using save slots though..) and it's so oldschool, and skill based, I love it.
It's an arcade-type side scrolling shooter, and you're a tiny spaceship, or something to the extent. The game takes you through the inside of some huge alien bug thing, first level being outside on the skin, then you go inside, and fight all the various things inside.
But be warned, this game is hardcore, the last level moves so fast that it nearly gave me a seizure. It's a rewarding experience though, and it's one of those games where I can say: Yep, I completed it. I'm pro. (with save slots.)