I liked the first 2/3 of Half Life, before it degenerated into jumping puzzle after jumping puzzle.

Half Life 2 is far superior in every way possible. The puzzles make better logical sense and are more consistent with the world. The varying styles of game play break the game up from being a grinding "shoot the crap out of stuff, reload" game. It jumps from being an FPS to a survival horror game to a driving game.

The physics engine is pretty cool too. I really like being able to think "You know, i could do this, i bet it would work" and it does work.

That dune buggy flips over way too easily though, andd it doesn't steer very well.