A worthy addition to the original Half-Life which is no small feat.
This time around you leave Gordon Freeman behind to do his business and you take over the role of Corporal Alan Shepard. One of the marines dispatched to clean up Black Mesa and find Gordon, that plan goes south pretty quickly once the aliens show up. Not to mention Black Ops has decided to enact its own agenda, taking out all who stand in their way.
Gameplay here is the same top-notch stuff found in HL1. The clever AI (namely its difficulty) is back here but instead of marine tactics, you have Black Ops kicking your ass. Gunplay is exquisite and is on par if not improved on from the previous game.
The game carves out a unique experience all its own with the addition of new weapons and enemies. The weapons now include a variety that Gordon never got a chance to use. Alien weaponry that are nothing but aliens that happen to shoot something, an increased assortment of standard military weapons, and just plain more of everything. New enemies dominate the game including variants of old favorites such as headcrab zombies that have evolved on step further.
Being a marine means you have marine buddies. These guys all have different roles from combat, to engineer, to medic. The engineer is needed to cut open locked doors (a scripted scene), the medic can give you health (think mobile health station), and you know, dudes that shoot guns at other dudes.
The main drawback however of the game is ironically the new weapons. You get swamped with the amount of guns you have. It can get tedious to go though your weapons to pick which one is ideal. It's harder to manage your stash of guns and like ammo this time around.
The environments are the cornerstone of the experience in Half-Life: Opposing Force. The levels are stunning, varied, and creatively different from the HL1's environments. Puzzles this time around are much better implemented and rarely do you go stretches without combat. Intricate and clever puzzles are usually associated with the amazing boss fights in the game.
The way the game flips the perspective from Gordon to the marines is actually pretty profound. It allows you to see the other side and give you a little insight. Playing through HL: OF, you get the impression that the marines you fought in HL1 are not really bad guys at all. They were doing their job, the fact that it was Gordon didn't matter. It goes to show that combat, and war really, is not personal. It's a job and in this case, a struggle to survive. You get this feeling like there's multiple sides to this (and all) struggle. This aspect of the story, given the effort to think about it, is just as effective if not more than the original game.
In short, buy this game. I will say stick to the forums and find solutions to any glitch you do find in the game that happens to exist depending on the version of the game.