It's an add-on disk. All the goodness of the original game, at half the price. Some of the new missions are pretty good. The plotline, involving a bunch of mutineers who (as it turns out) did the right thing, is good. (although I started missing the Concordia)
The new ship added, The Crossbow, is absolutely worthless. Sure, it's got a lot of guns, but it couldn't maneuver, and worse, it didn't have afterburners. Basically combining the worst aspects of a bomber and a heavy fighter, *it* was what made some of the later missions difficult. Give us a Sabre and it'd be easy. The term "sitting duck" barely describes it. Also, like the Secret Missions for WC, it only adds 16 new missions - as opposed to the 32 or so in the original WC2.
The Bottom Line
If you have The Kilrathi Saga, you can download it for free. Why think too hard about this?
Buy Wing Commander II: Vengeance of the Kilrathi - Special Operations 1