I am not going to gush over the Voom picture quality. I've been a Dish HD sub for almost 5 years, and never a Voom, so I can't compare the two providers. I can compare channels. I use a 921 with component outputs to a calibrated RPTV in 1080i.
The Voom channels are a lot nicer than SD content, and I'm very happy to have more HD content so I never have "nothing to watch", but I don't think the picture quality is up to par with HDNet or DiscoveryHD. Whether it is the mpeg encoding, the bandwidth, the resolution, or something else I'm not sure, but they just don't have that edge.
I've noticed a few times on Equator that the encoding just gets confused - there was some program with a large herd of zebras running from right to left. Consider thousands of vertical black and white stripes. The zebras in the foreground had visible stripes, those in the back of the picture had stripes, those in the middle blurred into a grey mass. I saw a similar problem with a flock of birds. Although I havent seen the same program on Discovery, I haven't seen anything similar there.
Another gripe I've got is that some of the programming is zoomed, so heads get cut off etc. I would rather see a 4:3 full image, if that's what the original source was, rather than zoom it and cut it.