I haven't used the HD version, but I do have the original which I used quite a bit on G4 and G5 system. It made h.264 encoding real-time on those systems.
I never liked the results as much as the ones I got from compressor, but they were fine for working with remote producers. There were a few more artifacts and less detail in the darks which the original Elegato seemed to crush.
The HD version seems to be much more customizable than the the original, but I get quite a bit better than real-time encoding on my intel machine and for my workflow I only encode at night, lunch, or break time anyway so I haven't looked into speeding up the process recently.
Jeremy Doyle
http://www.jeremydoyle.com