The first image taking in a subway is IOS 100 and given the available light even at f2.8 only produces a shutter speed of 1/6 showing motion blur in the people.
Raising the ISO level to 800 gives a respectable speed of 1/90 second in which I have a clear and sharp image.
Looking at the walls which are a neutral part of the images at 100%, it is clear that the amount of digital noise has increased and the second image at IS0 800 is much more grainier.
In concentrating on a darker part of the image it can be seen that there is more digital noise (speckles) in the second image, though I think the camera has done quite a good job here at reducing this and there is a good level of detail
The following 6 images also follow a similar pattern, the first shows motion blur at ISO 100 and 1/10 of a second but a good clean image, whilst the second at the higher end of ISO 1250 has a crisp image at 1/125 of a second.
However on investigation at 100% the floor can be seen to be significantly nosier using the higher ISO setting of 1250.
Finally , in the darker parts of the images it can clearly be seen that there is a lot more speckled digital noise in the high ISO image. The image at 100 is blurred but certainly no noise at all.
In conclusion using higher ISO values is a useful tool when shooting in low light where faster shutter speeds are required, but only as a last resort and always switch back to the standard 100 ISO once finished.