Hauptwerk does indeed have great qualities, but the main point is: MyOrgan is free.
With Hauptwerk, first of all, a hefty fee needs to be paid to buy the software and then it still costs a lot of money buying instruments for it. (I don't really remember how many instruments it comes with as standard, but I don't think there are many, and they are not exactly 'cream of the crop', ie. good, but not gigantic).
If I could afford HW, I might consider buying it, but only because I seem to have problems with MO, problems in the line of my ASIO driver on my soundcard (An M-Audio Delta 2496) not working properly with it, sometimes I have a crackle in my sound too, funny enough, I used to have a Sound Blaster Audigy, which is (in)famous for its crackle and pop, so I changed it for the M-Audio, and, indeed, many problems, eg. latency, sound quality, seemed to show a big improvement.
However, when using MyOrgan (with the Delta, that is), I started having problems with using the ASIO driver (which I did not have with the SB), and I seemed to get some crackle in my sound here and there, it has now improved having downloaded the latsest driver (just released) for the card, but it is still not 100%. I have sent messages both ways, ie. to the support people of M-Audio and to the author of MyOrgan, but both seem to be either reluctant, or unable (or both) to help.
A further problem I have is that I have lots of trouble getting the stops to do exactly what I want them to from my host MIDI application (Nuendo 3), the explanations on the Kloria web site are very vague about this, and I have changed the event for changing stops from a 'Note On' message to a 'Program Change' message, as the former did not send any stop change at all.
So, yes, if I could get those problems sorted, I would happily carry on using MO, even if only from a financial point of view.
They do say, 'never look a gift horse in the mouth' but obviously, if that horse has no legs, you are not going to get very far on it, are you?