A beautiful side story to the original Half-Life that shows the game a soldier's perspective...
Graphics: An actual improvement over those of its original game, with the HD models pack, it updates the look of the environments, character models, weapons, and sound effects (the best change being that they made the SMG into the M16A2 with an M203 attached).
Sound: as I said above, the HD pack that was included into this game increases the sound quality and gives the NPCs better voices while you weapons sound different. The biggest difference is the shotgun's reloading sound (sadly I noticed these kind of things...).
Gameplay: This game introduced more weapons that I feel should have been kept into Half-Life 2 such as the 'Squid gun' that shoots spore clusters that explode and blow anything apart, or the 'Shock bug' which similar to the Hive Hand, but shoots unguided electrical bolts. And more human weapons were added, like the clip-based magnum, the M249 SAW, and the M24 sniper rifle, a huge improvement over the crossbow. The multiplayer was even better in this version too, but good luck finding anyone to play it now a days, but thanks to steam, some people still actually do...
So if you own Half-Life, and have a Steam account, you should have this game, and you should have played it. Its worth the time to play it...