Some context first. I live in a 4 floor town house, built less than 10 years ago. My main tv is on the first floor (back of the house) and the tv I wanted to get the signal to is on the third floor (i.e. two floors above), also at the back of the house.
The box itself has HDMI passthrough, which is useful with a Sky box (but must be turned on for the passthrough to work). Connection to the tv on my first floor takes a little while (as others have mentioned), but I get decent signal strength. The picture itself is rock solid, as is the sound. I think it is every bit as good as that on my main tv, which is a nice surprise. My main tv is a 55 inch Samsung LED and the receiving tv is a 50 inch Panasonic plasma. On a tv that size I would expect any degredation to be obvious.
Compared to my old Digisender, well, there is no comparison. I am extremely pleased with the Gigaview 820. My biggest fear was whether my second tv would be too far away, or that the picture would be poor.
Other good points? It doesn't interfere or conflict with my wireless internet.
Bad points? Well, out of interest I did also try it on the other tv on my third floor (at the front of the house). It does not pick up any signal at all. Whereas my standard setup is pretty much vertical in terms of where the signal is going, the diagonal route to this other tv obviously increases the distance quite a bit. I'm not surprised it didn't work, but I think it gives some context in terms of the potential reach of the signal.