It should be a bit more efficient per-clock compared to Haswell, and also consume less power per-clock. Intel quoted the GPU performance improvements with 40%, which would probably translate to 20% real-word performance increase.
So all in all, nothing to get overly exited about, but I expect the Broadwell to be a next evolutionary step into closing the bridge between the mobile and the desktop.
Now, Skylake is when exiting things will start to happen. AVX-512, DDR4, Thunderbolt 3.0, mmm!