Usually I agree with KDR, but in this case I will make an exception. Half life 2 isn't your run of the mill RTS. There I said it. But, it isn't more then one by much.
I got the game, not because I was super stoked about it, but because I was curious. I wanted to see what the hype was all about. (Same thing with Halo 2, but I never actually did go buy an Xbox for it). I played the original Half Life, yes, but to a point. There was a point in the original *spoiler* that all your gear gets jacked. I was very put off by this, and quit. That was all I knew, and that was years ago.
What Half Life 2 has going for it, is that it doesn't spell anything out. It is the "experience". The plot, even after you beat it, is never fully finished. By playing through the eyes of Gordon Freeman, you learn everything he learns, and likewise. Anything you do, he does. It's an interesting twist, that is able to keep you in suspense longer then if you weren't inside his head.
Contrary to what KDR says though, there are 2 driving missions. I won't ruin the total number of missions for those still playing/wanting to play, but there are more then 5 times that number of regular type run and gun missions.
The gameplay in and of itself can sometimes be a bit shallow. Possibly it was the difficulty I was on, not requiring more, but I really only ever used 2 guns. (shotgun and magnum)
The characters they talk about, and the water to go with it, are both top notch. Best faces ever, and best water yet. Period. End of sentence.
Basically, if you would like to be wowed, perplexed, and definatly confused, play and beat the game. If you want a deep actioneer, look to Prime 2. It's that simple. Oh, but did I mention it's fun? That needs to be said somewhere. The last mission especially.
Now if you will excuse me, and hopefully forgive me for such a wandering post, I must go.