ANT531,
I don't really have time to go extremely in-depth at the moment, but all of the cards have strengths and weaknesses, except for the WinTV-HD which is totally outclassed by the other cards and not worth considering.
The AccessDTV has buggy software for the time being, but it's the only one that can pause live TV. The HiPix has lots of specialized features in the software thanks to the AVS Dev project, but there are still some bugs and the interfaces isn't quite as polished as the competition IMHO. I don't know much about the MyHD since it's the only card I haven't owned; all I really know is that it has firewire support for D-VHS, which I don't believe is fully supported on the ADTV or HiPix, and it also has analog recording if that's of interest to you. The HiDTV has firewire and analog recording, background recording, a polished UI, and in my experience has been the most stable of the cards I've used. The only real downside to the HiDTV is that it only has one tuner input; if you're planning to use the card for both OTA-DTV and analog cable TV, this could be a problem. But personally I don't think any of these cards are very well suited for analog TV, they're first and foremost DTV/HDTV cards.
I've owned 4 of the 5 cards, and currently own 3 of them. The one that best suits my needs is the HiDTV Pro, but I also use the HiPix in another machine for recording since I already had it and couldn't justify purchasing a second HiDTV for recording. My AccessDTV is not currently in use, so if you decide that's the card you're going to choose and you want to save some money by purchasing used, send me a private message and make me an offer.
If you have specific questions about a certain card or feature set feel free to ask.