I think it depends more upon the software as to what file type it saves. Anyway, that's just one more reason I like and use TSreader. Tsreader will save files as transport streams and can even split the files up into 2GB or smaller files and name them sequentially the same way the MyHD card does. therefore your MyHD card will recognise them and play them back just as if they were files it recorded. TSreader is not free and costs $99 or thereabout, but it's a very nice piece of software and the authour Rod Hewitt is working constantly to improve it through updates and that includes adding support for new cards. Also in addition to DVB-s cards Tsreader supports a wide variety of DVB-T, DVB-C, and ATSC (and perhaps even limited QAM) devices. That makes it a very flexible solution. For example, in addition to my Twinhann card I also have a Sasem OnAir usb and I can use it for OTA ATSC with my copy of TSreader as well. All in all, I'd say TSreader is well worth it's price and is a good investment for anyone interested in this hobby.
But I can't see any reason why you would want to do that. Every video card on the market has MPEG2 hardware accelleration, which makes the use of MyHD pointless. Even for HDTV.
Actually, while I normally agree with this philosiphy with regard to the DVB-s cards that have onboard hardware decoders, the MyHD card is somewhat useful in this regard because unlike the cards like the Nexus which only handle sd, the MyHD does seem to do an excellent job of decoding 4:2:0 HD transport streams at pretty high bitrates. As you say, most modern PCs can handle this also in software but at very high bitrates you might get some very slight stuttering in certain situations. Therefore hardware decoders like the one on the MyHD can usually provide slightly smoother playback and slightly better image quality. Of course this is only true when the hardware decoder is upto the task of handling such high bitrates. In the case of the MyHD it is able to handle HD 4:2:0 at bitrates upto somewhere around 35Mbit/sec and perhaps even a bit higher.
Another hardware decoder that can handle such high bitrate 4:2:0 HD streams with ease is the Roku HD1000 photobridge. It's a networked media playback device and has a built-in ATI Xellion GPU which handles the Mpeg2 decoding. I have one of these and TSReader supports streaming live video dirtectly to it. I use it all the time for watching 4:2:0 HD feeds and it works great. In fact the only time I don't use it is for 4:2:2, which of course it doesn't support. Anyway, a hardware decoder can be great, if it's able to handle HD at the necessary bitrates. The problem with alot of the older dvb-s cards with onboard decoders is their decoders aren't capable of handling anything beyond regular low bitrate SD, which makes them essentially useless in this day and age.