Time Crisis has never been known for its deep gameplay and complex strategies, but it's still a darn fun series and TC4 is the best one so far.
Time Crisis is basically a target game built around a lightgun, which was/is its major draw at the arcades. The name comes from the limited amount of time you're given to complete each stage; usually you feel like you're running behind the clock.
Sure, you're shooting "people," but there's no blood, dead bodies, exploding body parts, or other gratuitous violence found in many other FPSes. Honestly, I'm thankful for this.
Graphically it looks great, as can be expected of most PS3 games.
The classic arcade version runs "on rails"--your character is moved from position to position by the game, all you do is point, shoot, and RELOAD! (The announcer always screams "reload" for some reason.) TC4 adds the ability to look left and right in some scenes, a gun flashlight, and some interesting new opponents, but otherwise is pretty much the same formula as the earlier games--make it through each chapter without getting shot too many times and fight the boss at the end.
TC4 includes an FPS mode where you can move around using the Guncon's builtin joystick, but I found this frustrating and not nearly as fun as the classic arcade version.
TC4's Guncon is based on the "LED marker" technology used by the Wii and several other new controllers. Older Guncons used a video-sensing lightgun, but that method doesn't work with LCD/LED/plasma/projection TVs. The new design works with any TV, but I've found it is harder to calibrate and it really doesn't feel as accurate as the previous Guncons.
(Note that the new Guncon can't be used with previous TC versions, and old Guncons will not work with TC4.)
Time Crisis can be played with a gamepad, but it's a pain and I don't recommend it--moving the targeting cursor around the screen with a joystick is slow and awkward. The game's sole basis is really the Guncon; believe me, I've tried playing it without (as the old Guncons didn't work with my rear-projection TV....)
Should you buy TC4? For fans of the series it's an easy purchase, especially at the new low prices. If you're unfamiliar with the series, keep in mind there isn't much depth to it as there's no real strategy involved--it's just shoot, shoot, shoot. But for mindless fun and excitement it's hard to beat.