A game so nice, I bought it twice (don't mind the time played). One of the most fun shoot-'em-ups I've found on any platform. The fundamentals are rock solid, and their apparent simplicity will test you in surprisingly varied ways. Steam has a lot of shmups to offer, and you might wonder why to bother with this one... I'll try to spell it out below.
This game dispenses with novel mechanics and focuses on fun gameplay, level designs, and interesting challenges. It's got oldschool genes in it, but doesn't pigeonhole itself into being retro. It's neither horizontal nor vertical, but requires you to move and shoot omnidirectionally in every level. Horizontal scrollers tend to have a lot of environmental hazards, while verticals tend to have bullets flying everywhere... this has both!
Every level has unique situations to deal with, and even a veteran player will have to master them individually. Losing your powerups isn't the death sentence it can be in other shmups, because they aren't key to an easy victory - no screen-filling death lasers here to make you a sloppy pilot. There are enough powerups to mix things up, but they're random. It's a game of skill, not memorization.
Difficulty and pacing are outstanding. The curve is fair, practicing produces noticeable improvements every time, and when you accomplish something, you feel the glory. But it's also a game easily picked up and put down by the casual player. You're not stuck playing the same levels in the same order, and don't have to beat every previous level for another shot at one you've unlocked. Levels are like Starfox - 3 difficulty paths you can travel between as you meet requirements. All of this together makes a game well-suited for casuals, hardcore shmup fans, and anything in between. You'll always have something to work toward that isn't TOO far away.
Scoregasm is in my personal top 10 for shmups, and is my pick for best twin-stick game on the PC. Though made with the X360 controller in mind (I think), any Dual Shock works fantastically with it. If you like shoot-'em-ups, I can't imagine not liking this, unless you have some very specific preferences for your games.