What is the video quality of AT&T Stream Saver?

Stream Saver promises to reduce data plan usage for AT&T Mobility customers. But how does it affect streaming video quality on my device? How much does Stream Saver throttle video?

AT&T Mobility

AT&T Stream Saver reduces streaming video quality to Standard Definition (SD). This means that video streams are throttled to DVD quality, or approximately 480p resolution.