Mysteries Of The Sith is an ok game at best. Make sure you have a walkthrough handy though.
MOTS features 14 new missions, four of which you'll play as a somewhat aged Kyle Katarn who was the main character from Jedi Knight. The other ten you'll play as Maria Jade, a character from some of the Star Wars books. The story is standard Star Wars fare and is fun and interesting enough to keep you going despite the games shortcomings. The ending is not something that some players will enjoy but it does answer some questions that players might have had for the start of Jedi Knight 2.
The gameplay in MOTS is pretty much exactly the same as it was in Jedi Knight, which is a good thing as Jedi Knight was a great game blending first and 3rd person action amazingly well. But be warned that these missions assume you have played Jedi Knight as they are challenging from the beginning onward. However the difficulty really ramps up in the last 3 missions pitting you against creatures that can kill you in literally seconds.
The new levels in MOTS are great . . . . . . . . . when you know what your doing. The biggest problem plaguing MOTS would be that the level design is both quite inspired, and yet both terrible at the same time. Playing the game without a walkthrough is not recommended as you will spend a terrible amount of time just running around wondering what to do. Often you are left to search for one switch, or a grate you can slash open that blend into the environments just a little too well. In other cases you have to make seemingly impossible jumps that tend to rely purely on luck, or you using a cheat code. This slows down the flow of the game terribly and makes playing the game as frustrating as it is fun. The things you need to do to progress in a level are seldom obvious, leaving you to spend hours running around searching for what the developers wanted you to do. By the time you've figured out what to do (be it by yourself or with a helpful guide) you'll find that MOTS is also a very short game and can be played in about ten to fifteen hours tops. It's worth mentioning that MOTS does have a multiplayer function but if you can find someone playing this game online 8 years after it's release I'd be surprised.
Graphically the game is on the same level as Jedi Knight, making the visuals quite dated but still good enough to not seriously detract from the gameplay. The game features heavy use of right angles and most of the environments are quite jagged, the textures are not very detailed, and you'll find yourself running around a lot of big empty rooms. Sadly the cut scenes in MOTS were rendered using the game graphics instead of the live action ones that were used in Jedi Knight. As such I personally found them to be less enjoyable as they just looked terrible when compared to the cinematics of Jedi Knight.
MOTS however deserves honorable mention in it's sound department. The game score is comprised of recycled Star Wars music, which isn't a bad thing and it serves the game well. The voice acting is what really shines in MOTS, in particular the actor for Maria Jade does an outstanding job. The sound effects are all what you would expect of the star wars universe and everything was obviously mined from the films which like the music is not a bad thing.
So overall MOTS is an ok game at best, and unless you are a serious Jedi Knight fan you'll probably be better off skipping Mysteries of the Sith all together.