I think the latest softyware and firmware upgrade has done wonders for quality, especially when the bit rate of the connection takes a dive, the current recovery rate is quite rapid. Of course, I'm comparing this to a benchmark of how it looked just a month ago when I first got into Sling box.
Internet streaming video is really not about high quality at all. That's what HDTV broadcast is all about. The main purpose of internet video is to make the content, communication available to a wide variety of audiences, on demand. While PQ often suffers, it is considered good enough in most cases to be successful in this purpose. The internet video is best viewed on small screens, like PDA's and windowed laptops, certainly not 1080i FPTV on a 120" screen.
I do webcasting as part of my business and I can say it is very well accepted because the sales results from it are phenomenal. The only complaints I get are when the viewer's connection speeds dip too low and they complain about constant buffering. that causes hesitation in the playback. That's with my Video on demand service but the sling system is like live TV and as such MUST have consistent high bandwidth or it fails. The whole thing is still a compromise but if your expectations are realistic, you can be quite entertained by sling,even with EVDO on Verizon. I use that with my laptop and it works great with the new software version. Earlier versions, IMO, required wifi minimum.