[tmurray]

Desktop: high-end consumer GPUs have about 10-15x the single-precision FLOPs and 4-6x the bandwidth of a single Intel CPU socket. At this point, usually connected via PCIe Gen3. There are two real vendors (NVIDIA and AMD), and what comprises a system is generally the same (CPU + some number of GPUs).

Mobile: GPU has 3-5x the FLOPs of the CPU and no bandwidth advantage because of the shared memory pool between CPU and GPU. GPUs have very wide ranges of functionality. Even the CPUs behave very differently (Krait in Nexus 4 sometimes chews through code that the A15 in Nexus 10 chokes on and vice-versa). What comprises a system varies tremendously--CPU, CPU + GPU, CPU + GPU + other processors, etc.

A developer shouldn't be expected to have to tune for 20 different processors and system architectures in order to ship an app on the Android market. That's the problem we're trying to solve, not simply exposing access to GPU compute.