How much better HD audio is on Bluray over "legacy" codecs, is a very controversial topic amongst people interested in such things
Of course, the companies marketing these new HD audio codecs and those marketing the associated eqpt, would have us believe that the difference is like night and day!
My personal experience is that the difference/improvement is there, but it's often fairly subtle - I certainly don't hear the huge overall improvements others seem to.
(btw - I'm talking about full bitrate lossy codecs, not what you typically get on DVD or via Sky etc)
However, there are many variables at work - nobody else can listen through my ears, or with my eqpt, or in my room, so it's very much a personal thing!
As ever in such things though, placebo effect is ever present- we often hear what we want/expect to hear. It's so easy to be tricked - it is, after all, the basis of how all the lossy codecs work, and why blind testing methods are are always used in any study (not just audio testing), where preconceptual bias might be a problem (eg which cola tastes best?)
People can test this for themselves at home though - in a general sort of way. Try encoding a favorite high quality stereo audio track into various compression codecs - eg DD, MP3, DTS, AAC, OggVorbis, WMA etc, all at highish bitrates - can you tell the difference, and which is best? (as in tasting cola, different doesn't necessarily mean better).
And can you tell which one is best when you don't know which is which beforehand?
BTW - I've never heard a HD audio track sound worse than a "legacy" one, so there is the school of thought which says just pick the HD track, if you can, and be done with it - it's as good as it gets anyway!
Of course, the question then is, if you can't currently pick the HD audio track, is it worth the expense of buying new eqpt so you can?