Short version: For SWTOR fans, Revan will be a fascinating introduction to the Sith Emperor and a setup for Revan's appearance in the game. For KOTOR fans, unfortunately, Revan will be a disappointment.
Now for the longer version. There were two main things I imagine every KOTOR fan wanted to see in this book: (1) the story of how Revan (and Malak) turned to the dark side, and (2) the old crew from KOTOR at least make some appearances if not participate in the action. Unfortunately Revan fails to deliver on both, at least not satisfactorily.
Yes you do get the story of how Revan fell to the dark side, but it was executed poorly. Rather than show us how it happened, Karpyshyn instead has that event described through dialogue. Even beginning writers know the maxim "Show, don't tell", but unfortunately Karpyshyn didn't follow that tried and true advice. So instead of getting a vivid presentation of one of the most interesting moments in Star Wars history, we get a few lines of dialogue describing it. Bad writing choice, in my opinion.
As for the other characters from the KOTOR games making an appearance: one makes brief appearances mostly at the beginning and end of the novel, three rejoin Revan and are main characters of the novel, and the rest are only mentioned in passing. Now I had no problem with only having select people come back for the novel, but where I think again we have wasted opportunities is that the characters who do show up don't develop at all. Another basic rule of writing is that every main character in a novel should have some kind of significant internal change or transformation over the course of the story. Unfortunately, there's no real transformation for any of the KOTOR characters (even Revan, but to make my case I'd have to spoil things). Big things happen to them, but they essentially remain the same people throughout. It's fun at first, but eventually leaves them uninteresting.
The one character who does have an interesting development is the new character introduced in this novel: Lord Scourge. He is a compelling character because we see him mature as a Sith Lord as he navigates his way through the political labrynth of the Dark Council on Dromund Kaas. By the end of the novel he is a significantly different person than who he was at the beginning, and that is satsifying to see. If only the other characters had the same journey.
Ultimately Revan is worth reading for the SWTOR lore and for the Sith Emperor, who is probably the most terrifying and fascinating villain yet in the Star Wars universe (though it should be noted that the Emperor was Daniel Erickson's creation, not Karpyshyn's). Understanding the Sith Emperor sets the stage for what the story arc of SWTOR is really about, which can definitely enhance your appreciation of the game. To keep this review spoiler-free, however, I will have to leave it there as any further comments would require more detailed discussion.
EDIT: There is a good review on Amazon that explains in more detail about the disappointing aspects of the book, but it has some spoilers so be aware.
EDIT 2: Karpyshyn has written his own response to the criticisms of the book (he specifically mentions the Amazon reviews).
EDIT: There is a good review on Amazon that explains in more detail about the disappointing aspects of the book, but it has some spoilers so be aware.
EDIT 2: Karpyshyn has written his own response to the criticisms of the book (he specifically mentions the Amazon reviews).