Improves on Wing Commander but without adding anything new
Secret Missions was originally intended as a bonus for a special edition of Wing Commander that came with a baseball cap among other things. Origin decided to also release it as an add-on pack directly available from them mail-order and finally made it available at retail at Chris Roberts' request. It adds a new slightly smaller campaign of 16 missions to the original. Wing Commander was a great game so more of it could never be a bad thing.
Despite the slightly smaller number of missions, there is more gameplay here than the original Wing Commander. Assuming you were using a joystick, the original game was really a little too easy. I played through the whole thing in little over two hours. This took twice as long and the difficulty level was pitched about right. The missions here often require some sort of tactical approach above blowing everything up as quickly as possible and you may find yourself avoiding some combat or maybe just destroying your objective and retreating. For the most part, the missions tread the fine line between challenge and annoyance.
It would be a stretch to say that the original game had a running storyline. You talked to pilots between missions who maybe shared tactics or told you about some Kilrathi ace who just happens to appear in the next mission every time. There were minor themes that might cover a couple of missions but even the cutscenes showing the progress of the war didn't really have a direct relation to what you had been doing. Secret Missions definitely improves on this with a clear final goal throughout the game. In Secret Missions, you are chasing a Kilrathi super weapon (which destroys a colony at the start of the game) into Kilrathi space. With a bit of sidetracking here and there the game moves towards the destruction of this, with missions to stop it being resupplied and the like and it just comes across as a better told story.
The branching mission structure from the original game is gone. Here you just have the 16 missions and have to beat every one. If you fail there is a two mission retreat but you cannot win the game. From my point of view, this isn't much of a problem. For the most part I would expect players to keep trying a mission until they beat it, that's certainly how I play the game. This means I never get to see any of the other missions and they are pretty much wasted effort. They do add some replay value, however, if you go back and deliberately lose missions which is missing in this add-on.
There isn't really anything new here. Given the origins of the game this isn't too surprising but it is very much a case of more of the same. You start from the games worst ship and gradually work your way up to the best one just like in Wing Commander swapping between all the same wingmen on the way. All the enemies have been seen before with the exception of the final capital ship. Your wingmen are just as useless as ever and the opposition not really any better. To increase the difficulty level, the game resorts to throwing ludicrous numbers of enemies at you and you will frequently end up taking on 6 at a time in the final few missions. This shouldn't really be possible - I'd prefer to have seen fewer enemies with better A.I.
The Bottom Line
If you want more Wing Commander this fits the bill perfectly. No one who enjoyed the original game would be disappointed with this unless they expect something new along the way.