I actually saw Aliens before the original Alien, when I was still relatively young, and it was my first rated 'R' movie to boot. So I didn't believe my mom when she laughed about how obvious it was that the loader Ripley operates in the first half of Aliens would be used to in a big fight in the second half. Nor did I believe a teenager recently who claimed that Alien was scarier than its sequel, because just one unseen monster against a bunch of unarmed blue collars is more psychological than a horde of them against a team of well-armed space marines.
But looking at it objectively, they're probably right. Alien (directed by Ridley Scott) was meant to be scary. Aliens was meant to be a special-effects extravaganza of the sort only James Cameron could concoct. Alien 3 (properly written as Alien³) went back to the one-alien, helpless-humans angle, which won the approval of fans of the first film and the disappointment of fans of the second.
This time, the director was David Fincher, who (according to the IMDb) didn't have any major directing credits prior to this, although he was responsible for visual effects in Return of the Jedi and Indiana Jones and the Temple of Doom. But he went on to direct The Game and Se7en, suggesting that the psychological-terror angle is still something he enjoys. Sigourney Weaver reprises her role as Ellen Ripley, still the only character to survive the preceeding story and with even more hair cut off than in Aliens.
Other than that, the writers (yep, it's plural) kept with the spirit of the series by dumping Ripley and the alien in yet another completely new environment: a dead, barely habitable planet housing a prison occupied almost entirely by male sociopaths with British accents who claim to have "got religion" as part of their rehabilitation. (It was more successful for some prisoners than others.) While Ripley is defending herself against the prisoners and their warden and waiting to get rescued, an alien face hugger quietly hatches aboard her ship and impregnates a dog. The dog dies giving birth to an alien well out of sight of anyone else, and the alien proceeds to do its thing: quietly hunting and killing anything that it finds, one by one.
Which would have been fine for it, except that Ripley's exerience with the critters helps her organize the inmates to drive the alien to somewhere it can be killed. All well and good, except that being a prison planet, there aren't many weapons to be found to help with the job. And to complicate things, the nameless Company which has been coveting the alien as a biological weapon throughout the trilogy quietly gets a transmission of Ripley's medical data revealing that she, too, has an alien egg inside of her. And so the race is on to kill the alien so that Ripley can kill her own embryo before the Company arrives to take either one back to Earth for its own purposes.
Is it good? I'd say yes, it's as good as the first Alien and probably better. The story is too different from Aliens to fairly compare the two, but the one element they have in common is Ripley's character. The first film turned her from an ordinary "space trucker" to a self-reliant and inventive monster fighter -- usual scary movie stuff. The second turned her from a tough but post-traumatic woman to someone who can confront her own nightmare and win.
So now that she's both tough and in control of her fears, what do the writers do with her next? Answer: make her put it all in perspective. The prisoners need her to fight the alien that has hatched, while all Ripley can think about is the one that's inside of her. Her hatred of the aliens (and the Company) combined with the knowledge of her own impending death is keeping her from thinking about the future of the surviving inmates, and that needs to change before it's too late.
The only problem, story-wise, is that while this technically counts as character growth, it's really not very far for Ripley to grow. And while the inmates are all written as unique personalities, it's nearly impossible to identify with a bunch of imprisoned murderers and rapists regardless of how much they've changed or how much danger they're in. You can cheer for them, you can hope for them, but unlike the previous two, you can't really sympathize with them. This is a problem that no amount of writing can solve; it's written into the story and setting itself. When people speak ill of this film compared to the other two, they may not know it, but this is the reason why.
The film's greatest virtue is that it gives closure to both Ripley and the aliens, supposedly for good. Unfortunately, nothing is forever when writers don't own their own characters, and thus a fourth film titled Alien: Resurrection was eventually made, which proceeded to throw character development as well as common sense pretty much out the window in gratuitous pursuit of yet another sequel to a popular franchise. I've seen that movie but twice, in theaters and at home. I feel I owe it to the original trilogy never to see it again and just pretend it never happened.