All I can say is... Holy Crap.
Sound: Same as Half-Life 2. And that is in no way dissapointing.
Gameplay: Over-the-top action sequences are what Episode One is all about. Towards the end (which doesn't take long, more on this later) it gets to the point where I would describe it as rediculous (in a good way). There's no new weapons introduced in Ep. 1, but all the originals from HL2 make their way into your arsonal. There was, however, one new enemy added to the fray. The Hulk Zombie (not an official name, that's all me), which is, apparently, created by a Combine soldier getting zombified by a headcrab, is generally slow, but can charge fast, and takes a lot of damage.
Massive, chaotic shootouts are the highlight of Episode One, most of which beat the hell out any scene Half-Life 2 had to offer. From running through a field of debris, with manhacks in the air, mines on the ground, Combine soldiers charging from all angles, snipers in the windows, and a rocket launching tank on the other side (all at once), to a one-on-one duel with a strider, Ep. 1 will contantly have you on the edge of your seat, leaving your mouse dripping with sweat. The difficulty has clearly been bumped up a bit, too, from HL2. With noticeably smarter AI, and generally more enemies to deal with at any given time, Ep. 1 actually made me turn the difficulty down to "easy", something I never expected after playing HL2.
My only gripe about Ep. 1 is that it is very short. We're talking