One of the most prominent themes in these films (the better versions anyway) is a perceived dichotomy between individuality and the nasty things humans do. I believe that this dichotomy is a falsehood. That the road away from "zombiedom" leads BOTH toward genuine expression of individuality AND toward greater control over our dark sides. But despite this disagreement between myself and the creator of this basic storyline, I still find the corresponding films to be very engrossing. Something about the slant through which these films can explore: the nature of sanity/insanity, what makes people we love who they are, character reactions to their world rapidly becoming and alien one, etc. Long story short: I've recently put myself through a little marathon of variations on this concept. Here is the list of films I've seen, in order of preference:
1) Invasion Of The Body Snatchers 1978
Oh what a difference well-detailed characters make to a film like this (or most any film for that matter). Great attention to detail in general and a solid filter for keeping the usual Hollywood BS out of it. This is the only version that manages to do that without fail.
2) Body Snatchers 1993
Rather good on the character detail side. Minimal BS.
3) The Invasion 2007
Quite a bit of lip pouting and the like, but ultimately not an entirely wasted effort.
4) Puppet Masters 1994
Pretty damn useless.