A good try
The story taken from the "Space Vampires" novel is good. Having Dan O'Bannon write it made sense considering its similarities to "Alien," but his and Jackoby's script is often excruciatingly bad. The dialog often sounds like something that should be read, as with a stage play, rather than lines that work in a movie. Lines like "Good god, it's gigantic!" That belongs (maybe) in a novel and doesn't resemble anything real people would say who aren't winking and nodding to the audience in the process. If you're purposefully going for cheese, then okay, but I don't think they were. Further, there are way too many stupid moments where no precaution is taken by imminent victims who should know better. Would you do an autopsy or for any reason stick your unmasked snout right into the face of a desiccated alien that you have no clue if it spreads contagion or is even dead? And surely they have containment areas with thick glass windows instead of basement supply rooms lined with high grade chicken wire? There's a lot of dumb stuff like this that is at least far less noticeable in better films like Alien or The Thing.
Now onto the acting. Aside from Railsback, who I don't think makes the grade here, the performances are pretty good. The British military intelligence guys and scientists add some credibility to the whole thing and ask intelligent questions, even if they too seem to think it's reasonable to stand 3 meters from an alien being that they know is going to explode into a ball of dust right in their faces. O'Bannon and Jakoby wrote some good dialog for them, but often put them in ridiculous situations inconsistent with their apparent intelligence. I especially liked Peter Firth's acting as Colonel Caine and the distinctly British manner in which he cut through the bullshit with his precisely worded inquiries and sharp observations. Both he and Michael Gothard (what a name for someone seduced by a vampire) were both very engaging and gave the film much needed acting credibility.
Finally, the special effects. At the beginning of the film I was thinking "Oh boy, I'm in for a real stinker." The scenes with the Churchill crew flying into the comet and exploring the alien vessel (Alien style) were, even by mid-80s standards, really amateurish and unconvincing. Alien did it right 6 years earlier. Couldn't they? But after that the special effects were terrific, except for the vampire lady bursting into Railsback's sleeping quarters. The animatronics of the creatures and their transformations were superb. John Dykstra and company did fabulous work here that deserves comparison to Rob Bottin, Stan Winston, Rick Baker, Chris Walas, and the other greats horror fans like I have admired. So, aside from the woeful outer space effects at the outset, Lifeforce deserved it's Academy Award for special effects.
In the final analysis I can only give this film a 6/10. There are just too many dumb moments with bad dialog and incomprehensible actions, albeit interlaced with moments of very intelligent sci-fi and grade A special effects. What kills me is that these transparent flaws could easily have been fixed by: (a) a screenwriter more capable than Dan O'Bannon, like Bill Lancaster, and (b) a more skillful director than Tobe Hooper that would've put the kibosh on some of the dumb scenes presumably put in by the screenwriters. Given the production values and the talent involved, this film should've risen to heights of Alien, The Thing, Invasion of the Body Snatchers, and The Fly, but some of the boys in key positions fumbled the ball once too often. A good try, but not good enough to stand with the giants.