HD homerun gives you the ability to capture HD recordings from the QAM signal (essentially, your cable line has HD without the cable box - you can get stations 2.1, 4.1, 7.1, etc in HD for the major networks). The HD homerun lets you record those channels.
It will not let you record 702, 704, 707, etc, in HD. Which is only a point of interest because you can get the major networks without the cable box, but NOT TBS HD, RUSH HD, etc.
It's hard to explain over this forum.
I'd have gotten a Tivo HD but didn't want to pay $10/month or whatever it is. looking back, that might have been a better way to go, but I've been using my media center PC for watching Netflix "instant" movies (basically streaming, it looks good), Hulu (also looks good), and YouTube. So that's good.