I have ESPN3 on my computer and on my VuNow through PlayOn. I have been watching the World Cup mainly on the computer on a HD TV and I have to say that the quality is really poor. ESPN3 seems incapable of maintaining a decent quality stream for more than a couple on minutes on any of their archived games. It is better than nothing but not by much.
If ESPN3 was brought to the Roku with the low quality I am seeing it would give Roku a VERY bad image in the sports watching world.
The quality might be suspect but I find, in general, the video quality far exceeds that of my other option for live access to sporting events.....the radio.
With no cable TV, I actually listen to a lot of games on the radio, unless they are on ESPN3 or OTA.....so yeah, it's not the greatest video, but it's better then the radio.