Lots of numbers from running cairo-traces on a i5-2500, which has a Sandybridge GT1 GPU (or more confusingly called HD2000), and also a Radeon HD5770 discrete GPU.
The key results being the geometric mean of all the traces as compared to using a software rasteriser:
UXA (snb): 1.6x slower
glamor (snb): 2.7x slower
SNA (snb): 1.6x faster
GL (snb): 2.9x slower
EXA (r600g): 3.8x slower
glamor (r600g): 3.0x slower
GL (r600g): 2.7x slower
fglrx (xlib): 4.7x slower
fglrx (GL): 32.0x slower [not shown as it makes the graphs even more difficult to read]
All bar one of the acceleration methods is worse (performance, power, latency, by any metric) than simply using the CPU and rendering directly within the client. Note also that software rasterisation is currently more performant than trying to use the GPU through the OpenGL driver stack.
(All software, except for the xserver which was held back to keep glamor working, from git as of 20121228.)