As the first game I played from Valve, I can confidently say, as cheesy as it may sound, that it was a hell of an awesome experience. But to not simply say the game is perfect, I have to mention that there were some parts in the game that I found REALLY boring, like how the maps are so huge and you end up wandering around for HOURS, often feeling bored just roaming without feeling like you're actually progressing in the game, since it takes so long to move from one area to another.

And don't even get me started on Xen, I have no idea what happened with that level, because while the beginning is just amazing, at some point you become the damn electrician for the aliens, and it's so damn annoying having to plug in the sockets like FIFTY times that I really thought about giving up on the game, but I persisted, and the result of that was really awesome.