Hi, I don't know the specifics on that site but HD downloads can be really excellent. There was a series of write ups on it in Stereophile earlier this year. They concluded (these guys are really analog oriented) that it was now good enough for them. They see it as do I as the way things are likely to go. With 24/192 and higher formats fed into a good DAC the sound is really hard to tell from an analog source. Another thing that bears on the subject is that nearly all music recorded for several years now has been done on digital equipment. Yes there is a resurgence of all analog recording and mastering, but it is quite limited in scope. Worse many of the LPs made in the past were from digital tapes.
Thus, my suggestion is on services like the one mentioned is to try one selection and see if it meets your demands for quality. If so great, if not, don't use that service again.
As an additional thought, I would not bother to get any HD digital material unless I had a very high quality system to play it on. The quality of the end result will be strongly influenced by the equipment playing it. A cheap DAC feeding a typical AV system such as seem to be the norm now will not be able to fully do justice to the HD recording. If your gear came from Walmart stick to iTunes. You will save a bit of cash.
Bruce, thanks for all the information. I particularly agree with the last part! I know a couple people who look for quality recordings but don't pay attention to quality in their sound system. It's puzzling.