So I thought I'd try out the Amazon per-episode experience with the new Top Gear USA. Regarding the content, it seems that TGUSA is steadily improving from episode to episode. I've seen the HD History Channel version, which is a very nice quality image, so that's my baseline.
The two Amazon "HD" versions I'm trying here are the live-streaming version (2.5mbps) and the downloaded version (6mbps). Since the downloaded version was (not surprisingly) taking a while to download, I started watching the streaming version and was VERY disappointed. It may technically be 720p, but I at first thought it was only a standard-definition stream, as the crisp sharpness of HD was lacking! Looking into things, I found that the streaming is only a 2.5mbps datarate, while the downloaded version runs at a datarate almost 2.5x that! We'll see how the downloaded version looks, but I'd certainly want my money back if the streaming version was it.
The downloaded version looks better, but it still doesn't seem up to the quality of broadcast HDTV or Bluray standards. If you're a big fan of this show, skip Amazon and watch it on History Channel.