On the HD channels it's pretty good quality it's comparable to netflix HD bitrate i think in 1280x720.
There is also another plugin called rtmpgui+ which does pretty much the same in a more "legal" coded manner, but allthough supertv hijacks some keys, use more CPU and stuff, I found in my experience to have most succes with Supertv anyway it seems to be a lot more persistent and consistent. (though I'm sure it's imagination) Heh..
Mikey1234 has also built a plugin based on favorite?/working? streams and they also float around on NaviX.
I this sometimes when my favorite channels are down because it has a good overview to find something else.
But defenatly a good plugin unless when to many people are on I can get some bufferunderruns but it's really not that often. I downgraded my cabletv because they requested insane amounts for 40 sh!tty tv channels, and since I could not cancel it (all people in the building pay for it so it would get more expensive for them if you could :/) I was forced to take the smallest cabletv package.
Anyway the channels I really missed was in this plugin. In HD plus more. So I actually use it quite some.
All 3 incarnations are on http://hub.gosub.dk/