Reply from Sony:
"The video quality of your movie is dependent on your internet connection speed. Sony recommends at least 1.5Mbps for SD and a minimum of 4.5Mbps for HD for the optimum experience. Qriocity is delivered via “streaming”, therefore if your internet connection speed changes during playback the smooth viewing experience may be affected. Although Sony tests your connection speed prior to playback, and deliver the highest quality video based on these results, your connection speed can vary during playback due to many factors."
So it seems as far as the company is concerned, as long as your connection never dips below 4.5mbps HD playback should be fine. As I said earlier, for me it always has been fine when my connection's been solid, but I'd be very interested in feedback (good and bad!) from anyone else who's used it.
Cheers,
Tom