I joined Prime years ago when it was just a shipping deal. When they added free video streaming, it had enough stuff I was interested in to get me to add a Roku to my theater - and extend my internet connection to my projector-based theater for it.
There's so much stuff available both through Prime and otherwise thru the Roku (check out Classical TV, which includes jazz and blues) that I'd never get rid of it.
The quality of 720p through the Roku is highly variable, from dismal to great. For an example of the latter, check out Firefly
from Prime - not far down from Blu-ray in quality - but not while I'm watching, please!
I chose the Roku because it is one of the few devices that gives me 5.1 DD from Prime, mostly on movies, but the Firefly
tv series is carried by Prime in 5.1. (My AVR is pre-HDMI, so I got a first-generation XDS for its optical audio output. I use 5.1 analog for Blu-ray.)
The addition of a lot of PBS stuff was a boon, but I'm mainly a music and sci-fi guy - also check out Pushing Daisies
(decent 1/2 HD) and Farscape
(the latter in SD) both from Prime.