I own both the US and UK versions of this film.
I rented the US version before buying, and since my expectations were so terribly low, I wasn't disappointed by the PQ; in fact, I was mildly surprised by the level of detail. This is likely because so much of the first ten minutes of the film consists of very tight close ups. Sharon Stone being featured in just about all of them didn't hurt either, lol.
My initial impression was that the blu ray looked A LOT like the film I saw in theaters: it was fairly dark, the colors were not terribly saturated, and there was a bit of dirt evident as well.
When I BOUGHT the movie, I noticed the dirt, the specks, the white spots, the hairs. But these appear primarily, if not exclusively, in the first 5 min. of the film. There is also some obvious and distracting fluctuation in the brightness level in different parts of the frame due to the optical effects work I would assume.
These problems largely disappear after the first five minutes, leaving you viewing a blu ray based on a print that looks much like the original theatrical release.
Also, the notion that you are getting dvd quality with the blu ray is completely erroneous. Despite the numerous issues with picture quality, level of detail is quite good.
The UK release is much, much cleaner. You're no longer looking at a dirty print and the brightness fluctuations are largely gone. Whether this is because the print was physically cleaned or whether it was remastered using an algorithm is unclear to me.
The UK release also receives a big boost in contrast as well. The colors are more saturated as well, such as the red on mars. There are some strange issues with color accuracy; Quaid's skin looks green-ish when he is working at the construction site. In other words, there are strange variations in color and contrast, but given the choice between this cleaner version an the darker, duller, dirtier US version, the UK version is definitely better.
I spent a little over $20 total to get both versions, so I didn't invest that much. The film is still enjoyable regardless of which copy you buy.