SamsonD:HUGE difference!!! It is like Dolby on steroids.
Its more dependant on the decoding amp actually. Some guys went to dolby to test out the latest HD audio and compared to uncompressed PCM (they were the same, although one guy managed to tell a difference which is intriguing). They then compared to a standard dvds dolby digital, and the differences were VERY subtle.
This is down to the fact a state of the art decoding amp was used as opposed to a bog standard one most people use. I have a marantz 8003 pre-amp and can also testify the differences are quite subtle (The pre amp had an approx 2.5k RRP new). Im not saying theres no difference, but when using a top notch decoding amp, the differences are NOT 'night and day'