Well, I have to say, I just watched the Director's Cut, and it's a much better movie than the theatrical release, even with only about an extra 12 minutes of running time and some swapped-in scenes which leaves about 20 minutes of "new material".
It's kind of amazing to me just how small tweaks / alternate versions of scenes, occasional extra lines of dialogue, etc., can make such a significant impact on the quality of a movie. It's Len Wiseman, so it's certainly now Shakespeare, but I think the quality has definitely improved to bring this movie into "watchable" territory now.
This particular release brings the movie up enough in quality that at this stage one's liking/disliking of the movie will probably be more based on whether or not you happen to like Len Wiseman's movies rather than because the movie is just a dog all around. if you hated UnderWorld, chances are you still won't like this; if you happen to like the Underworld films (which didn't fare well from the critics POV but still have their cult following), the Director's Cut might be enjoyable.
For whatever reason, the original movie felt "bare" and just entirely action-oriented, whereas this version inserts just enough dialogue back in where the larger issues of reality vs fiction seem to be more addressed. There's even a shot near the end where the entire reality of the movie is purposefully thrown back into doubt.
Another correction is ensuring that it's clear why Lori continues to try to kill Quaid even after having orders not to. A few ideas here: (1) She is pissed at Cohaagen for sticking her with a guy of his caliber without warning her effectively, putting her at risk, (2) professional jealousy, and (3) she views him as a traitor -- she believes he actually IS a traitor to the government when trying to kill him, so she doesn't care what her orders are. Cohaagen never really tells her what the master plan is; and when the major reveal occurs, Cohaagen threatens her and tells her he'll deal with her insubordination later. I don't recall what appeared in the theatrical version, but it was very clear to me in the director's cut.
I think the largest "correction" of the movie is putting Ethan Hawk back into the film. The premise here (and it is specifically noted by the dialogue) is that Quaid's appearance has been surgically changed. This helps explain why people do not recognize him on the street or elsewhere. I only wish they had had more of Hauser in the movie -- in particular, instead of having Cohaagen doing the "big reveal" in the rebel base, having Hauser do it via hologram. That was probably one of my favorite parts in the original movie, and it was disappointing not to see it here.
The movie itself is as beautiful as ever on BluRay. Underworld was all done in shades of blue; this is done in Blade Runner grit, but there are still vibrant colors of all sorts, including reds and yellows. Wiseman does frame scenes decently, and I never really had trouble following the action. I listened to about ten minutes of the director commentary near the Extended edition, and it's clear that Wiseman does think through his scenes. In the portion i listened to, he was describing how a lot of people use all CGI nowadays but he doesn't prefer having his cameras do impossible things, he likes to use it more realistically. And when they do the motion-detecting suits, for characters who actually are wearing body armor (or the robots, for example), he doesn't like to use skintight leotards because people move different depending on the weight and style of what they are wearing -- so he would build those limitations into the motion-detecting suit itself rather than just adding everything on in CGI.