This game is pretty cool. Diehard 1 is a very different game. It reminds me of Resident Evil, but the camera follows you almost exactly like that 1st run SNES game, NCAA basketball. It is pretty cool, with lots of blood and killing. Diehard 2 might be my favorite of the games, just because with the HyperBlaster gun it is very, very cool. Without the gun it sucks though, so I suggest buying the gun when you get the game if you can afford it. Diehard 3 is also very cool. It is a driving game, with the angle similar to Twisted Metal, only cooler looking. It is cool because the levels change, so its not always the same thing. You start off in the city ramming into bombs, then you have to race a dump truck through the subway, then you get to drive through central park killing stuff.
Overall it is a good buy, and each game in itself is pretty cool...but for 3 games, definately worth it. The blood and gore may not be quite as good as Project Overkill, but it is still very bloody. Part 1 is the most gorey, then 2, and 3 with rather minimal gore. (but in 3 you can run over pedestrians, so it still rules.)
One nice thing is the game only takes up 1 memory card slot, yet you can save 8 games for each Diehard. Also the game is not really easy, so it makes it more fun to play.