Usually you have an option to download Amazon titles to your PC which will get you 720p at 6 Mbps with 364 Kbps 6 channel sound (what I got for the titles I own, all HD television episodes that I somehow missed recording). 6 Mbps for 720p is massive as bit rates for streaming video goes, though I don't know that I'd consider those encoding impressive.
What I have been impressed with recently encoded titles streamed through my Roku 2 XS at less than half the bit rate. I've been watching Fringe over again and it's pretty crisp, Though not comparable to BD, about as good as HDTV. The basic DD surround sound is also nice.
EDIT: I just watched that newly-transformed-Captain-America-chases-the-German-spies-on-foot scene and the train crash scene in Super 8 and the sound rendition for both was very good, though I'm sure they don't rival the lossless tracks on the BDs.