Mysteries of the Sith was fairly decent in a time when shooter expansions were typically poor.
The storyline in MotS is... uh... well... it's non-existant. Instead of giving you one fleshed out plotline the game is split up into four "story arcs" that span multiple levels. In the first story arc which is set seven years after JK you play as Kyle Katarn as he defends a New Republic base on Altyr V from killer Imperial Remnant asteroids. In the second story arc you play as Mara Jade as she gets involved in a feud between two hutts when all she wants is to negotiate a New Republic trading request with one. In the third Mara is babysitting a New Republic cruiser which is attacked by pirates, and Mara must then recover a Jedi holocron from the pirates. The final (and best) is where Mara investigates the swamp world of Dromuund Kaas to find the Sith temple that Kyle went to investigate, and never reported in from. None of these plotlines are really explored, but the final one is very atmospheric and definitely the best of the four.
MotS plays just like JK so the only new gameplay elements are the levels, some new weapons, and some new force powers. The levels like in JK are very well designed, and some of the new weapons are nice, but some of them (like the Carbonite gun) are just plain pointless. The new Force Powers also seem relatively pointless. MotS at least tried to add more than most FPS expansions.
The graphics of the new levels and enemies are nice. But the new enemy models still have that blocky feel that is a hallmark to the JK engine. The new "colored lighting" effects are very nice, though, and add a surprising amount of atmosphere to MotS' large sprawling levels.
The audio is nothing special. You have typical Star Wars sounds, decent voice acting, and once again the John Williams score on the disc in CD audio format. It's not bad, but it could have stood to have more to it.
Mysteries of the Sith is a light in an age of terrible shooter expansions. Granted MotS doesn't surpass JK; it still manages to deliver a fresh and satisfying experience as well as some memorable moments in the single player mode that are bound to please any shooter fan; Star Wars follower or not.