This is about as perfect of a shmup as I've ever played. It's not too easy and not too tough. The bullets fill the screen enough to be challenging, but you don't feel like you are getting hit unfairly. The speed of the game is brisk enough that you don't find yourself frustrated with the controls, nor is it so fast and slippery that you can't manage to finesse your way through a wall of bullets. It feels like the perfect challenge level, though maybe that just means it's a good match for someone of my personal skill level. It has a semi-overhead/semi-3D scrolling background similar to Axelay, but the gameplay feels a bit more akin to Gradius because you have several supporting characters that work sort of like the "options" in Gradius. I have no idea what the story is about since I don't speak Japanese, but there are flying cat girls, so it's probably nothing mind blowing and the pictures will be enough to keep you entertained. This is a free download, so give it a try. I highly recommend this one.
Strike Witches STG Website
Direct Download Link
Gameplay Video