They add pretty much nothing, really, and nobody's doing it right anyways. Killzone 3 looks like a pop-up book.
Then you are doing it wrong. I've been using nVidia 3D vision technology and it is fantastic. I'm not talking the crappy red/blue classes here, I'm talking the shutter glasses. 120 htz dividing the frames up for each eye (effectively making it 60htz each eye which is the norm). actual 3D modeled games (which pretty much all new games are) are certainly not over lapping 2D images at different depths. If you see that, you haven't set your computer up right.
Now some people can't even see the 3D (no two brains are alike and they work in mysterious ways) and for them I say, since you can't experience it, you really have no opinion on how it affects gaming experience.
It would be nice if they worked with this. Not only is 3D for the pc, now with 3d tv's taking over the scene. Oblivion in 3D was absolutely astonishing and Skyrim is supposed to have even better graphics over Oblivion.
I can see it being no big deal though...for Oblivion I had to turn down water and glass detail to get rid of shadowing/ghosting and the 3D didn't work so hot with shadows so I shut them off (all of which increases performance anyway). I saw two 'crosshair' icons when not focused on it directly but the geforce technology comes with its own crosshair (a page of different styles to choose from).
The shadowing/ghosting, I'm afraid is inherent to my setup (monitor maybe, I don't know for sure) but the problem with shadows has to do with the game (The Witcher 2 has shadows that work just fine with it).