I can't say much about the popularity of GPUs in practical calculations.
From a development point of view the speedups that can be expected from GPUs in plane-wave DFT are only moderate, probably around 2 to 3, maybe 7 if you are optimistic. See for example the paper describing the VASP implementation  or
this stackoverflow question illustrating speedups of 2 to 3 for the key computational step in plane-wave FFT, namely the fast-fourier transfrom (FFT).
This might not sound so bad, but keep in mind the extra requirements in computational power, which is also about a factor of 2 or higher, the price of the GPU (on top of a CPU you need anyway!) and the added complexity inside the code, which might well prevent the implementation of other faster algorithms in the future. With that in mind, I would say GPUs have potential for large-scale cutting edge calculations, but are not able to change the picture much over CPUs for the type of calculations you describe (a few days or less): You might as well buy another CPU rack and just use that on top.
 S. Maintz, B. Eck, R. Dronskowski. Speeding up plane-wave electronic-structure calculations using graphics-processing units, Comp. Phys. Comm. 7, 1421 (2011) DOI 10.1016/j.cpc.2011.03.010.