Hero Core was fucking sick.
I loved the combination of schmup and Metroid-like exploration and can't really think of a game that plays quite like it.
Think there are a few twin-stick shooters that are similar though, but I like that you can only shoot in two directions in Hero Core. Makes it feel like a classic schmup where you have to position yourself to get a good shot at the enemy.
Never beat the hard mode though, shit got a bit too bullet-hellish for me. But, I will always recommend this game to any shmup fan.