I just got the UK version for the PQ. Last night I compared it with my bought-on-release US HD DVD. The differences are well noted in the screenshots already posted, and I could see them clearly when swapping still frames between the UK & the US, although on my 53" they are a bit harder to notice in motion. However, owning the UK version, there is a certain peace of mind that you're watching the "best" version for PQ, though! 
What I was surprised by, not having played the HD DVD for a while (and presumably these comments apply to the US blu as well) was the improvement in the US editions over the UK regarding the presentation of some special features -
* Director's Notebook is simply a series of featurettes in the UK version, whereas the US edition offers you both an selectable Index like the UK, or you can flick through HD pages of the Notebooks, with clickable links to watch the featurettes. This is a nice enhancement of the material.
* Comics are HD, and animated. UK presents "comic sketches" which look SD and are actually different entirely to the US content.
* Posters are obviously HD and much clearer on US disc, look muddy and SD on UK
* There are a bunch of HD galleries of creature design, production design, and production scrapbook. UK only features a creature gallery as a hidden feature. Again, these look SD compared with the sharp images of the US disc.
US exclusives -
PiP track (can't comment, never watched it)
Charlie Rose show (48 mins)
Extra US trailer & 8 TV spots
Animated Comics
3 Production Galleries
UK exclusives -
Del Toro Guardian interview at the NFT
DVD comic sketches
While I'm glad to have picked up the UK blu for the PQ, I think when wanting to look at the extras, the US set has more elegant menus and the enhancements I listed above. I thought I'd post this as I don't remember ever reading about the differences in presentation of extras on the discs before.
What I was surprised by, not having played the HD DVD for a while (and presumably these comments apply to the US blu as well) was the improvement in the US editions over the UK regarding the presentation of some special features -
* Director's Notebook is simply a series of featurettes in the UK version, whereas the US edition offers you both an selectable Index like the UK, or you can flick through HD pages of the Notebooks, with clickable links to watch the featurettes. This is a nice enhancement of the material.
* Comics are HD, and animated. UK presents "comic sketches" which look SD and are actually different entirely to the US content.
* Posters are obviously HD and much clearer on US disc, look muddy and SD on UK
* There are a bunch of HD galleries of creature design, production design, and production scrapbook. UK only features a creature gallery as a hidden feature. Again, these look SD compared with the sharp images of the US disc.
US exclusives -
PiP track (can't comment, never watched it)
Charlie Rose show (48 mins)
Extra US trailer & 8 TV spots
Animated Comics
3 Production Galleries
UK exclusives -
Del Toro Guardian interview at the NFT
DVD comic sketches
While I'm glad to have picked up the UK blu for the PQ, I think when wanting to look at the extras, the US set has more elegant menus and the enhancements I listed above. I thought I'd post this as I don't remember ever reading about the differences in presentation of extras on the discs before.
