As I was expecting the original game was good, but not great. The engine is very pretty, the physics are spot on, but it just doesn't quite hold together very well. It felt like I was playing lots of separate sections which were loosely tacked together by a story. First was the intro bit, then some sewers, then a boat ride, a Resident Evil type bit, a bit where you drive round in a buggy, a prison, city fighting, and then a futuristic alien citadel. Individually there was nothing wrong with any of the sections, but there wasn't much feel of continuity between them. The other main gripes are that the weapons all feel underpowered, there isn't much variety of enemies, and every puzzle is a physics one which requires the gravity gun. Apart from that though, a decent enough game.
Episode 1 took things downhill. Very short, no new enemies or weapons, and not a great deal of story behind it. Just as things seemed to be getting going, it was all over.
Episode 2 however saved the day. The game felt a lot more like a coherent universe, the story was much better, and there were some really nice set pieces. There was even a bit of psychology going on - I was convinced at multiple points that I was about to be betrayed by Magnusson. Subtle hints had been dropped the whole way through, and getting to the end and finding that actually hes a good guy after all was a refreshing change from the usual cliched plots that we normally get.
I tried a bit of TF2, not sure if I'll get into it though. I've become accustomed to Quake Wars recently which demands a bit more thought and cunning than your average fps, so it seemed like a bit of a backwards step for me but we'll see