The "paranoid thriller" is one of my favorite subgenres so I've always been a big fan of the original Invasion of the Body Snatchers, which has a surprisingly potent paranoid atmosphere for a Hollywood movie made in the 50s. But it has an infamously unconvincing "happy ending" that I don't think anyone who sees the movie really buys - luckily, two decades later, Hollywood was under no compunction to tack on such a happy ending. By the time Philip Kaufman's remake came around, depressing was the new happy, and disturbing was the new palatable.
I still prefer the original, but it's really hard to argue against the subtle, creeping dread of the remake, particularly in the first scenes. So few movies, particularly mainstream Hollywood genre ones, have the courage to just put weird, unsettling stuff on the screen without calling attention to it or explaining it. But this movie has enough weird stuff for a dozen movies, with Kaufman doing everything he can to put the audience off-balance, from weird camera angles to a shot of Robert Duvall as a priest playing on a child's swing. Whatever works, I guess.
The area where the remake suffers by comparison to the original is once the danger becomes explicit and it devolves into a long chase between Donald Sutherland and his slowly-decreasing band of human friends and the overwhelming population of pod people in San Francisco. In the original, this part of the movie begins and ends in 10 or 15 minutes, whereas the 70s remake is bound by the expectations and conventions of a horror/thriller and has to provide the requisite amount of "action." Unfortunately this stuff is the least satisfying part of the movie compared to the subtly haunting atmosphere of the first 2/3rds.
Fortunately, however, this is still a much more special movie than 90% of horror movies out there, and it picks up again in a big way in the famous final scene. I guess I should also mention that dog/folk-singer hybrid, which is still one of the most disturbing things I've seen in a movie.