Hey Retro_SA, I feel that there's a good number of quality Mame light gun titles that justify a light gun purchase. Some notables are Sharp Shooter, Police Trainer, Point Blank 1/2, Bang!, Egg Venture, Lethal Enforcers I/II, Crypt Killer, Area 51/Maximum Force, Wing Shooting Championship and Trophy Hunting : Bear and Moose. In addition, I have the Virtua Cop 1 & 2, House of the Dead I & II and Big Buck Hunter. I have two Act Labs guns and I think they perform pretty well, although one of them works better than the other. I'd like to have a recoil/force feedback option with the guns. I haven't tried any other PC compatible light gun.
Like DJ_Izumi said, the PS2 is the console to have for light gun games (Time Crisis 1-3, Vampire Night, Point Blank 1-3, Ninja Assault...). The Dreamcast also has some good titles. I used to have a single cab w/all 3 in a single cab for my shooting fix. Good luck with whatever you decide.
DJ, if you read this, I actually purchased a component cable with Guncon input, but it doesn't work. The best quality (clarity-wise) I was able to find with a Guncon input a s-video cable, which looks better than the composite cable. May I ask where you found your Guncon-compatible component cable?