I didn't buy the whole Quadrilogy because I don't care for Alien3 or Alien: Resurrection, but I did buy two-disc DVDs of Alien and Aliens and I think the documentaries for both are pretty awesome, some of the best DVD special features that I know of. I've proudly held on to these over the years and wouldn't want to not have those docs around. The one for Aliens of course details how it was the first really awful time James Cameron had giving birth to a movie. If you didn't know this, he's said that The Abyss was the biggest hurdle he'd had to get past in his career.
The alien in the 1979 film was a tall African guy wearing a costume and most of the aliens in Aliens were dancers wearing Danskin outfits or something like that, that's what I recall from the documentaries. It's for the best, yes, that we can't see them very well. Wasn't the initial skirmish with the aliens and the marines supposed to resemble a Vietnam battle?
Dialogue in Alien? It's mostly servicable but probably peaked with Ian Holm's severed head warning Ripley about what was going to happen. Holm's performance is hugely underrated, I think he's a great villain! As for good lines...uh...."Why don't you just f*** off" from Ripley to Yaphet Kotto and Harry Dean Stanton? I suppose great lines weren't really what that film was about, Dan O'Bannon was mostly concerned with mashing up old sci-fi movies to make something new.
What do you think of Alien: Resurrection? The only thing I like in that movie besides Weaver is the over the top gory death of the hybrid alien which is so sick minded as to be kind of funny. I think most people agree the Winona Ryder character was a failure but much of the "defense" seems to be people who never want to admit Joss Whedon does anything wrong.