I bought one (VoodooTV 200) Friday, returned it Saturday.
The video image on it was terrible, it flickered and wavered like a poor VHS tape. The software was buggy and slow. The video capture acted like it was working, but wouldn't create files that were valid and readable by any any other video player program (neither MPEG or AVI files worked), and when I tried to use the Voodoo software (called VisualReality) it locked up and I had to kill the program. Also when trying to capture to uncompressed AVI file, the file size was exactly the same as a compressed MPEG capture of the same length, so it obviously wasn't outputting uncompressed video.
Other sites have reviewed it and not had problems, but I thought it was terrible. I went and got an ATI TV Wonder and have had no problems with it, and the video is near perfect (sharper than on a TV).
I did test the cable signal by connecting it to a real TV, and it was fine. Maybe I had a bad card, but...the Voodoo does use a digital tuner chip instead of the usual analog tuner like ATI and Hauppage use.