I'm running one of these on a cable modem with 1Mb upload. You can view any resolution over this connection, but the frame rate slows down to what your connection can support, and playback may become choppy at high resolutions. My HD camera offers me 240p, 480p, and 720p via mydlink. Only 240p plays really smoothly. Others are more choppy, but the frame rate isn't shown. The mydlink website doesn't display the actual frame rate or bits per second (the frame rate you get may be less than you configured in the camera if your network bandwidth is insufficient).
The mydlink site also displays the image smaller than actual size (less than 800x1280), so the HD doesn't buy you as much for live streaming. Even my DCS-942L is shown smaller than actual size. The HD camera does appear larger than the SD camera though. To get actual size, you need to use the advanced setup function to connect directly to the camera, or connect directly without mydlink (requiring more skill to gain access through your router's firewall)..
The default resolution on a cellphone is much smaller than 720p. My default resolution is 320x176. This gets about 20fps via wifi (may be limited by my phone, Android, not a current high end model), less on my 3G data connection. My phone only lets me increase to 480p. At that image size, I get about 2-5 fps via wifi, and a bitrate peaking around 100 kbps, which is less than my cable modem upload speed, but possibly all my phone can handle.
Note also the free mydlink tools impose a viewing time limit on my phone, I think this is 5 minutes. The mydlink website does not.
HD is useful, but mainly for still images, or video saved locally for viewing later. It also works on your local LAN, but won't likely support 1250x80 at 15fps over your 1Mb internet connection.