3d in games is not something I've ever paid attention to.
Do you need a 3d capable lcd like with tvs or can regular screens do it? I've got a 480 and got some 3d glasses at home from a trip to the cinema recently.
So far as playing PC games only nvidia offers 3D, but I read that ATI will be doing something too in the future.
For the nvidia system you need a 120Hz LCD monitor, or a 120Hz capable DLP projector. A projector is on my to-do list in the near future!
http://www.nvidia.co.uk/object/GeForce_3D_Vision_Requirements_uk.html
It is true that the frame rate in games will be halved, but one thing I've found is that in 3D you can run at a lower resolution without noticing the pixels. Whatever the reason I was playing Metro 2033 last night at 1360x768 and the pixels were no more noticeable than in 2D at 1920x1080.
If you already have a compatible 120Hz screen (unlikely) then for just over £100 the Nvidia system is easily worth it IMO. Of course whether it's also worth the expensive of a new £300 screen depends what you want from your games.
In my opinion it is a gimmick, but in a good way. I don't often play games but when I do I enjoy the story and above all the atmosphere. This is where the 3D helps as a well designed environment just comes to life in 3D.