Worth it. Just worth it.
Don't take me too serious because I am a hard-headed fan of Syphon Filter. I have played all the previous SF and enjoyed them all. Even Omega Strain.
Dark Mirror brings up a whole new story with whole new enemies and weapons. Finally you get to use special goggles, one's that detect body temperature (to find enemies, it helps on stealth missions), one's that indicate important mission items or objectives and of course the famous Night Vision goggles.
After you complete the story mode you can play some bonus missions as well as access a panel that contains how many enemies you should kill with a knife, with the stealth rifles as well as how many guys you should kill by using environment means, or how many guys you killed in stealth. This is probably the only thing that makes the game re-playable after ending the story mode.
Graphics are pretty amazing as usual and sound effects are better than ever.
The gameplay itself hasn't change a bit. You still have a bulletproof vest that you can magically switch as well as your surprisingly enduring Health.
There are no obligatory stealth missions but in many of them, maintaining stealth will save you a lot of trouble, although you might think twice before doing all the missions in stealth since that will make you lose most of the fun.
Now the big issue: No multiplayer.Not even one like Syphon Filter 2 that allows you to bring a friend and fight against it.
Unless you are a hardcore fan like me, you'll probably wonder if all the money you spent was worth it.