Having now seen the DC, I can say that (a) seeing Alien on the big screen at long last, after having been considerably too young to catch it in 1979*, was splendid (and it didn’t hurt that the print and sound system were in first-class shape); (b) it was interesting to see the alternate cut, with the deleted scenes restored; and (c) I’m really glad the upcoming DVD will still contain the original theatrical cut (in addition to the DC).
With the possible exception of the “listening to the signal” scene, I thought all the editorial changes in the DC were for the worse. That said, I thought that most of them — everything but the insertion of the “cocoon” scene, in fact — weren’t significantly for the worse.
I don’t think the film is ruined by the majority of the changes: it’s just made slightly less effective. As nice as the Lambert/Ripley spat outside the infirmary is, for example, it takes away from the focus of that scene on the horror of what’s been done to Kane. Same with the longer/re-edited version of Brett’s death scene: it’s not bad, by and large, just not quite as fierce and shocking as the original edit. (It also gives you an earlier glimpse of the Alien, which I think weakens the horror: as originally edited, we get our first look at the full-sized monster as it unfolds silently behind Harry Dean Stanton, a great frisson shot.)
The “cocoon” scene, by contrast, even in the considerably-abbreviated form in which it’s included, really does stop the film dead in its tracks, just like Ridley Scott’s been saying for the past 20-odd years. Reinserting that scene seems like an odd way for him to prove the rightness of his argument, though. :-)
Anyway, I’ll be pleased to have the various deleted/changed scenes in fully-restored and -produced form on the new DVD. They’re interesting scenes in their own right, and they make nice companion-pieces to the film itself. But I don’t think I’ll watch the DC again except out of academic interest (comparing sequences between the two cuts, etc.): if I just want to watch the film, or show it to someone who hasn’t seen it, I’ll stick with the original release version.
* = Hell, I couldn’t even make it all the way through the damned thing pan-and-scanned on tape in 1984, when I was 14: I think seeing it in the cinema as an eight-year-old in 1979 would have left me a traumatised wreck. (Okay, more of a traumatised wreck.)