Out of all the PS2 games this one has my favorite progression system. Weapons be purchased, leveled up and upgraded individually, armor can be upgraded, max health can increase - there's a lot. You combine that with everything else and a solid gameplay loop and it's easy to keep coming back to this game. The humor is really strong as well and there's a good deal of variety. I will say my main complaint is that some levels of the main campaign are more memorable than others.