Opposing Force shouldn't be held on a pedestal the way Half-Life is, simply because Gearbox (the developers of this one) stood on Half-Life's already established shoulders in order to reach even higher. It's hard to deny though that Opposing Force is more consistently fun than Half-Life is, and doesn't have Half-Life's low points, like "On a Rail". Xen makes several reappearances, but it's only used sparingly for effect, which is how you do Xen properly. Awkward platforming sections are kept to a minimum. The new weapons are super fun, the new enemies are creepy in all the right ways, and the level design is top notch. You can tell they were full of ideas for this game, and there are quite a few new features and concepts crammed into the game's comparatively short playtime.
It doesn't quite have Half-Life's level of polish, and it lacks a certain subtlety that Half-Life had, but it's a great game absolutely worthy of the name.