HL2 uses a very well designed game engine, the physics are as near as can be matched to real world physics and the gameplay reflects this.
The graphics can be set to run on anything from a GEForce 2 through to a Radeon 9800 or above with equal enjoyment.
I enjoyed it, the only drawback is that the single player missions plays like the first part of a book, and you are left wondering when you get to find out what is going to come next. It is not conclusive in anyway.
Doom 3 kicks HL2's butt when it comes to sitting alone in the dark with surround sound and a 21' monitor scarind the pants off yourself, HL2 beats Doom 3 for general playability and everall enjoyment.