I don't think Batsugun annoys the purists, but it does annoy me as it's BLOODY HARD. Yeah, it's a great game, but I like to play my games with a single credit, and I could never get past level 3 with one credit.... alas, I can barely beat level 2 boss, I usually get there wih 5 bombs, and I need to waste them all to beat it
Raine is THE SHMUP emulator, if you use it, get the Cave games, as they are great. Donpachi, Dodonpachi, Dangun Feveron, ESP Rade and Guwange, all fantastic games..
In Raine there's also Armed Police Batrider, one of my favs. All those games plays a lot like Batsugun, and are rather better too (not saying Batsugun is bad, it indeed is a great game)