A marked improvement on its predecessor, yet still dragged down by week spots.
You'll be able to explore the new setting in a largely unchanged fashion, the graphics, voice acting, rule set, character development system and party interaction are the same as in the previous title. That's hardly a detraction as they were all excellent. The Sith Lords however manage to improve on many of them.
The story and characters are probably the strongest features of the game. The setting is decidedly darker and more ominous this time and contrary to the original title, the story is very interesting, full of surprises and unexpected twists. Your actions will again visibly affect the world, and your companions who can be swayed by your example to the Light or thrust to the depths of the Dark side of the force. The characters you'll meet are well portrayed, especially Kreia, who will constantly challenge your every action, questioning the very meaning of good and evil. Often times you'll be forced to realise that there is no black and white and that even the best of intentions can have unforeseen consequences.
The authors added a lot of welcome improvements and nice touches to various aspects of the game. For example, you'll no longer crawl at a snail's pace while in an enviro-suit or the AI will alternate with you for the starting turns in pazaak. You'll also meet several heroes of the original and revisit places you travelled to before, further strengthening the continuity of the two titles.
Unfortunately, nothing was done to address the greatest annoyance of the Knights of The Old Republic - the low difficulty. Indeed, the problem became much more severe this time. Your character will begin as a Jedi with access to the force and you'll be able to use new feats, powers, lightsabre forms, prestige classes and equipment making you an unstoppable powerhouse very early in the game, even on the highest difficulty. To be fair, the game is not always easy, at several times, you'll be forced to play with secondary characters and these parts may be very hard to impossible depending on at which point in your travels you encounter these. The encounter with Twin Suns is a good example, if you visit Nar Shadaa early in your travels, Atton is definitely in for one hell of a fight. Someone really screwed up the game's balancing this time.
The game also feels unfinished on many occasions - the T3M4's secret messing with the navi-computer, the HK-50 factory or the throwaway travel of Mira on Malachor all leave you feeling content was cut from the game at the last minute. Technical problems crop up more frequently as well, even with the latest patch, going as far as making some quests unwinnable, others completing immediately upon receiving. It's a shame, a couple more weeks of QA would have done The Sith Lords good.
Bottom line: This is basically a must play for RPG and Star Wars fans alike. A huge world, amazing, dark and ominous storyline, interesting characters with their own agendas, top-level voice acting and a cinematics feel to visuals won't disappoint, save for the questionable difficulty setting.