GTX 970: It was revealed that the card was designed to access its memory as a 3.5 GB section, plus a 0.5 GB one, access to the latter being 7 times slower than the first one. -- https://en.wikipedia.org/wiki/GeForce_900_series#False_adver...
Source: http://www.anandtech.com/show/10304/nvidia-announces-the-gef...
You may not notice it nowadays because several games received patches and newer nVidia drivers limit the memory consumption to just below 3.5 GB. Otherwise you would experience a major slowdown. Also a major problem for CUDA.
Just because not everyone writes CUDA applications that need access to all 4GB does not mean that people are using it wrong, or that they should mind that the last .5GB is slow. If it works for others' use then that's great for them.
No doubt that with future games the 970 will not perform as well as it should, but I will have a different card by then. What they did is a shame and shouldn't happen again, but I haven't noticed any real-world ramifications yet.
Even with 3.5gb it's still a great cost/performance card.