In the world of film, sequels tend to be pretty poor. They usually involve the same thing again, with the same characters doing the same thing, only set in a different location. What we have here is a very rare breed, a sequel that develops and expands the ideas of its predecessor and adds its own identity without losing the core ideals.
The best way to sum this film up is that it takes everything from Aliens and makes it bigger. Never mind one Alien, you get one hundred this time, along with their very big mother. Never mind a few lashed together flame throwers, this time you get full on military hardware. Yet despite all this, you still get the vulnerability and desperation of a rapidly diminishing group of people fighting seemingly unstoppable opponents. You also get the human side, as Ripley once again becomes the reluctant hero and shows her mothering instincts. The grim 'used space' feel remains, as does technology being no substitute for human instinct and ingenuity, along with the tension created by an almost unseen but tenacious foe
In fact, this is a major reason why the movie works so well, the Aliens are only ever glimpsed and despite all the firepower, for every one killed another takes its place, so technology is of little use. But what technology; power loaders, dropships, smart guns, turret cannons - set design is superb. The motion trackers especially are inspired; watching little dots getting inexorably closer proves a masterclass in tension.
The script is razor sharp (too many quotable lines to count), acting excellent and the balance between story and action is perfect. Is it better than Alien? Maybe, but in truth they are both brilliant in their own ways. This is how sequels should be.