I got 75-85% GPU utilization with ~2-3% CPU core utilization - it's ok to browse internet but watching movies is unbearable. Even with all that "superior" UVD3 of HD7k. But hey - F@H has the very same problem - as it is always when most work is done on GPU alone. Everything else become low priority and so desktop may appear sluggish.
What is weird - I'm getting flooded with the very same WUs, I actually did already some 30 tasks and there were only 3 or 4 different WUs - that means I crunch over and over the very same WU ten times or so... when I look at WU properties quorum is set to 2 - so I guess something is terribly wrong here because I send dozens of results for one WU and I guess other users too - than means few hundreds or even thousands of results per WU. I guess GPU can err much more frequently than CPU, but hey... few hundreds or thousands results to get validation?

I did only after your post. Number is so long and both first 8 digits and last 10 looks the same. Key is what's between them I guess (4 digits - so hard to notice on first look)

edit: I notices that task end up on 99.415% within first 3 minutes - then GPU utilizations goes to 0% and nothing happens for another ~1min, in that time CPU utilization goes 100% (one core, not whole) and I guess data from GPU is collected and "postprocessed". So for 3.2GHz Athlon II that means 3min of GPU work + 1min of CPU work versus ~90mins of CPU only work. Nice speedup, but in that rate we'll complete all GPU WUs very quick. What'll happen then? :)

It maybe the GPU you're using. I can run GPU WUs and watch a 1080p high frame rate video at the same time with a HD 7870. And that's running 5 WUs at a time.