Up until recently, I have been very happy with the picture quality of the HD, and especially the HDX titles. The other night I rented a movie called "Brotherhood of Blood" in HD, and it was absolutely the worst picture quality I have ever seen on my screen. This title was worse than VHS, and I highly advise it be avoided. It was un-watchable for me, I FF through to see if the PQ improved, but no luck.
I am a bit puzzled, although I understand it was probably a really horrible source content, you think it would be screened, and downgraded at least to SD. I really feel ripped off especially since the charge was $5.99.
Now I am hesitant to rent, unless I am really sure of a title. Directv, allows a couple minutes preview, before you commit to buy. This allows time to check and make sure that the aspect ratio is proper, and the picture quality is decent. Vudu should offer the same option.