First of all , DO NOT buy it unless you have AT LEAST a radeon 8500 or geforce 4 ti 4200 128mb, FX doesn't play it faster that 15 FPS!!
The hardware needs are an understatement . Yes the graphics are amazing! but the peformance to FPS ratio is bad . I get 20 FPS average on 1024x768 , texture detail high , particles off and shadows off . I'm dloaing new drivers, should help me a bit but still , it need MACHO hardware just to play it above HL 1 graphics .
The first level is remeniscent of HL , exploding passages , people shooting nmies for you etc . But the similarity ends there . The other levels are hard on hardware , whole oceans to render and miles of hills etc . Fun to use vehicles .
IF you want to play it on a geforce 2 you'll get headaches for the rest of your life , textures on low and medium are worse than HL .