I have the newest HD Slingbox and with 4g service the picture is outstanding.
I have two different inputs I can select from, standard cable and an HD cable box. If I use the standard cable input, a viewer at home can be watching a different channel than I am watching but the picture is lower quality.
I even get perfect TV reception while driving down the road in my truck. My Wife watched a movie in HD HBO on demand while we drove on a 3 hour trip last week.