Just as an example of the change this entails for deep learning, the recent "Exploring the Limits of Language Modeling"[1] paper from Google used 32 K40 GPUs. While the K40 / K80 are not the most recent generation of GPU, they're still powerful beasts, and finding a large number of them set up well is a challenge for most.
In only 2 hours, their model beat previous state of the art results. Their new best result was achieved after three weeks of compute.
With two assumptions, that a K80 is approximately 2 x K40 and that you could run the model with similar efficiency, that means you can beat previous state of the art for ~$28.8 and could replicate that paper's state of the art for ~$7257.6 - all using a single P2 instance.
While the latter number is extreme, the former isn't. It's expensive but still opens up so many opportunities. Everything from hobbyists competing on Kaggle competitions to that tiny division inside a big company that would never be able to provision GPU access otherwise - and of course the startup inbetween.
* I'm not even going to try to compare the old Amazon GPU instances to the new one as they're not even in the same ballpark. They have far less memory and don't support many of the features required for efficient use of modern deep learning frameworks.