It is the best value out there. With the proper settings and a 300 dollar pc, there is no issue for most setups..if you try to run it on a wireless under powered laptop with a bunch of 3mp cameras you will have problems...
depends on your encoding settings for BI when your not using d2d. Could be bigger, could be smaller. Hard drives are so cheap now adays its not really an issue anyway. Regardless of of file size, your loosing quality on the video file by re-encoding. Again, depending on your settings this could vary from not noticeable (but still there), to nasty. if your camera is streaming out h264, your likely not gaining a size advantage unless your downsampling your video in BI. may as well just set a lower bit rate in the camera and go to d2d in that case.
