There is definitely a noticeable difference. I have the HD add on for Xbox 360 and have been blown away by HD movies, not only that but it is by far the best upconverter player I have seen (I also own the oppo which is good but not as clear as the 360 add on)
With the death of HD-DVD and the prices in movies still not going down I think i'm going to dive into the blu ray market. Just having trouble finding a good deal