The step up in audio is HUGE even on a simple receiver based system. Once you hear the new codecs or even MCPCM you will not want to go back to lossy DD and DTS. It's sort of like going from .WAV or AIFF to MP3.
It's one reason why I'm so thrilled that BD (with a 100% track record for lossless from Sony, Disney, Fox and MGM) won out. Hopefully Paramount, Universal, and Warner will step up with lossless on EVERY high-def title.
Upgrading to my Marantz 8002 HDMI-equipped receiver was a revelation. For years I had remembered how much better the PCM on my laserdiscs sounded compared to the "MP3" sound of Dolby Digital on DVD. Now that wonderful "laserdisc sound" is back... this time in full 5.1 with a 1080p picture.