PSN sucks for shmups I'm afraid. I bought the gundemonium collection and it seems okay but haven't dropped alot of time into it. It's certainly unique.
soldner X is a cool game but I can't get into it. I hear the sequel is great and it definitely looks fun.
the PS1 import store has alot of decent shmups but nothing special I don't think, the only one I own is shienryu and I only got it for something to play on my new psp during family christmas shit. Seems like a good game, I imagine the saturn port is better but I have yet to reach a point in the game where there is any slowdown.