I Am Legend, the third remake of Richard Matheson's 1954 novel, is in wide release and I saw it last weekend. For me, it embodied everything that is both good and bad about SF movies. (Warning: the following discussion is a spoiler if you don't know the story and plan on reading and/or seeing it).
The good is 1) the riveting special effects and 2) the chance to correct whatever was shaky in the original. Two examples: Robert Neville is now a genetic scientist instead of a random Joe, which makes his medical research into the cause of the plague (and in the movie, a cure) much more plausible. Also, in the movie the dog that contracts the plague and dies is not just a stray he domesticated for a few weeks but his own dog, which makes the animal's death much more affecting.
As for the bad -- Because movie makers want both those sensational special effects and a happy ending, they are willing to do intense damage to all logic. In the novel, Neville locks down his house at night and listens to the once-human-now-vampirish, infected creatures howl outside. In the movie, they don't know where he lives, and once they find out, they tear the place apart because these starving, emaciated, human bodies can scale sheer walls, exhibit superhuman strength, and other absurdities. Worse: In the movie, eventually Neville discovers a cure and dies passing it on to another survivor with natural immunity. She takes it to a secret colony of survivors in Vermont, and this gift makes Robert a "legend." What are they going to do with this cure? The survivors don't need it, and to use it on the "zombies" you have to strap them down for 24 hours and drastically lower their body temperatures, a daunting procedure. Also, there are no infected zombies in Vermont or they would have scaled the pathetic wall that encircles the colony, an idealized New England town complete with fall foliage and a white steeple, and eaten everybody. In the novel, by contrast, the zombies capture Robert and kill him, and he realizes just before he dies that, to them, he is the nightmarish killer, the outsider -- the "legend." The worldview turns inside out, like a sock. It's a satisfying and unexpected conclusion which the movie junks completely.
There have only been a handful of SF movies (as opposed to fantasy) that I like, for just these reasons. Is it so impossible to ask for both a dramatic story and some basic logic? After all, print SF does it all the time.