I tend to agree with most of the reviewers here. While JK was revolutionary, Mysteries was okay.
The idea of not being Kyle, and in fact, having to seek down Kyle and stop him from inflicting mass destruction, is quite interesting. You also got to use different force powers from Jedi Knight, but most of these, useless. The game also adds a few additional weapons, the most notable here, being the electroscope, that allows you to zoom in on your enemies and snipe them. This is great for multiplayer players who use guns and want to be snipers.
For the saberists out there like me, though, Mysteries adds very little, though stands as a great game in its own right, complete with a good storyline. The visuals make use of colored lighting, and adds a nice layer of icing to the 3D cake. People have designed levels that look far nicer in Mysteries than they do in JK, though I've seen some amazing JK levels as well.
If you enjoy JK and you want another adventure, go ahead and pick this up! It's like $9 now, and was worth the $20 I paid for it.