I don't know if anything will change for the upcoming season, but I've had Game Rewind for the last 2 years. For both years, I could start streaming the Bills game (or any other NFL team's game) on demand shortly after the Sunday night NFL game ended. The stream was unavailable until then, and was also unavailable during the live broadcast of each Monday night NFL game.
So if you live on the East Coast, and work some type of morning hours on Mondays, the option of starting to watch the Bills game roughly around midnight Sunday night isn't very practical. On the other hand, if you live on the West Coast, or plan to delay sleep until you are dead, there is a viable window to watch that day's Bills game on demand shortly after the Sunday night NFL game broadcast ends, even if you have to work on Monday mornings.
The replay video is HD quality - - but the quality of what you personally get to see depends on the strength of your particular internet connection (just like for any other streaming video).
As I mentioned, I haven't seen anything yet about the upcoming season, so it's always possible the availability of the stream might change this year.
Hope that helps.