I started collecting the genre about four years ago. It is finite, but tons and tons of stuff out there, and the occassional new release every few months, which is nice. The last time I played a shooter before that was probably Axelay and Gate of Thunder in the early nineties. It was my favorite genre during the 16-bit gen. But I missed everything from 1995-2007, so no real history with PSX/PS2/Saturn/DC/etc. Been slowly picking up titles here and there, maybe have about 100 or so. Most I've not played yet or even tried. Mainly stick to my favorites which get good rotation: Gradius Rebirth Wii, Mushi Futari, Donpachi PSX, Espgaluda II BL 360, Raiden Fighters Aces (favorite shooter release ever), and more recently Mushihimesama.
Working on completing my full Saturn shooter collection, about 20 more titles to go. Starting a PCE Hu collection next. Got a few neat games already on Hu (some old favorites and some new titles). Gonna 100% that collection over the next few year as well.