I've noticed that some titles do not have Multi-Channel surround sound which is a little sad. My impression is that Vudu encodes/converts the movies to the HDX format from files that are provided to them. So it may be the studio's fault for that. Also it could be possible that they only received a SD quality copy but then digitally up-converted it to create the 1080P copy, much like a blu-ray player would do. This could explain some of the quality issues in the video/aduio if that was the case and would be a bit miss leading to advertise it as 1080p, if it was only SD upconverted to 1080p...
I really don't know if any of that is true though.
However, I wouldn't judge UltraViolet's quality yet.... The current providers, flixster, vudu and etc... are creating there own versions of the film for streaming. The common file format (CFF) for ultraviolet movies has not been officially released. These files are going to be standardized like the DVD and Blu-ray was for audio/video specifications. I would expect to see the files available at the end of this year or early next year. The files should provide consistent quality between any provider (vudu, flixster, studio site, etc).
Personally I would never recommend buying digital movies only. The reason is, what if UltraViolet closed shop and now your left without your movies in the future? That aside, I have converted almost every film that I have using D2D (only blu-rays) because it is cheap and convenient. I also purchase and redeem all movies that have UltraViolet codes and will pretty much only be buying movies that contain a code or can be D2D (unless it's just a must have film). My reason for this is because I see the potential in the system to remove the work out of having to maintain a server, htpc and other components to essentially build my own network of movies/tv with correct metadata. UltraViolet is convenient to use and the metadata is typically correct because it was done by the studio/provider. I don't foresee the system going away but I still believe it's best to always buy physical copies of the movies and TV. This way if the system closes shop, I can simply go back to ripping my own media and building my own library like I've previously done because I have all the films/tv on disc (can't do that using other providers such as apple/xbox video/etc).
Also, I tend to watch the movies and shows that I buy on disc only through Vudu or a UltaViolet provider with the exception of 3D movie releases. I recently watched "A friend for the end of the world" last week in HDX and was really impressed with the quality. I also picked up a copy of Abraham Lincoln Vampire Hunter in 3D and redeemed it, but I'm going to watch it in 3D via the disc instead of streaming it.