I have this device. Summary: For $20, it's an awesome deal, since I'm pretty happy with having spent $60 for it (granted, like 2 years ago).
Most questions seem to have been answered, but my summary:
1) Gaming: I played through most of Odin Sphere using a PS2 connected through this to my laptop when I was waiting for most of my stuff to arrive during a move. It's a Core Duo 1.86 GHz (not Core 2 Duo), and it worked fine. I didn't notice any lag, which isn't to say there wasn't any--Odin Sphere isn't exactly the most demanding game on precision twitch reflexes. However, occasionally the sound, oddly enough, would lag by a noticeable amount. You don't need an ATI video card (I have one, but this device doesn't care).
2) HD Recording: As others have pointed out, since Composite and S-Video cap out at SD resolutions, you can't record HD on those inputs. However, you can record at least 720p HD over the coax input (input from the antenna, cable box, DVD player/VCR, etc.) I verified that I had an old 1280x720 recording from this device. I don't know if it supports 1080p, and I'm not sure where you'd get the signal from, anyway.
3) OTA Signal Quality: There are online resources (www.antennaweb.org) that can tell you where your local stations' transmitters are. If they're more than about 15 miles or so away, I wouldn't count on being able to pick them up with an indoor antenna like this one.
4) Cable: If you subscribe to cable/satellite, you should be able to take the output of your cable box and route it to the coax input on this device and watch/record it on your computer. If you stop subscribing, you will no longer have this ability (though over-the-air broadcasts will still be available).
5) Recording Games: No idea whether you can go over 30 fps on a game recording, but why would you need to? For playback/passive viewing, 30 fps seems like plenty.
I think that covers most of the common questions I saw.