I have the add-on glasses on that I use on my Creative TNT2 Ultra. First, they only work in DirectX. No OpenGL. The effect is pretty cool. I think they're best suited to non-FPS games. In games like Half-life, the crosshair is sitting in 3D space, which makes it almost impossible to aim well. Plus in FPS you're looking for all the frames you can get. I never tried them with it, but I bet the Need For Speed games look pretty cool.
When you have the 3D driver enabled, you can turn 3D on and off with a single button at any time. You can also bring up a menu to control all sorts settings for 3D.
Performance. Enabling the 3D driver seems to knock off about 15-20% of performance. Then when you actually switch it on, it knocks off another 50% for obvious reasons.
Also, you get ghosts. The phosphors in the monitor don't fade instantly, so you can see ghosts of the other eye's view. (if you close one eye, you can still faintly see the other eyes perspective) This may be more or less noticable on different monitors, but it was kinda annoying on mine. Any bright lights would cause really bad ghosts.
Last downside: it knocks down the brightness quite a bit. You'll have to either turn up the brightness every time you play 3D, turn off the lights in the room, or live with darker gameplay. Turning up the brightness makes the ghosts more evident, too.
Anyway, the main downside is the ghosting. They make it harder to focus correctly and make the effect less believable. And this still may be mainly due to my monitor.
You really need a fast monitor, too. 75hz is not that great. 85hz is ok. 100hz is good. I never tried more because I don't think my monitor goes higher and it was difficult to set the refresh rate. Refresh rate changes doesn't seem to affect the ghosting problem.
The real Elsa drivers (the ones I can't use) supposedly have really good refresh rate control.
BP6, C-366/550 (x2 very soon), 320meg
CL TNT2U 180/230