In lieu of the traditional review preamble, here's the answer to the question you're probably wondering about: Yes, Aliens vs. Predator 2 lets you save anywhere. In fact, the first game is so notorious for its end-of-level-only save system that the sequel has become the first shooter to proudly list its unrestricted save feature right on the box. Monolith, taking over the series from UK-based developer Rebellion, has also added a few other things that the first game lacked, such as a compelling campaign structure, an in-game server browser, some interesting multiplayer modes, and the ability to play all the various Alien life stages. About the only thing it managed to screw up was the unimpressive single-player demo released a couple of months ago. So in answer to your second question: Yes, Aliens vs. Predator 2 is better than its demo. It's better than the demo, it's better than the original Aliens vs. Predator, and it's one of the best action games released so far this year.
The first game relied almost exclusively on moody lighting to support its somewhat simple-looking environments. The sequel, while not needing it so much as a crutch, continues the trend of fear-inducing lighting schemes. Red security lamps, strobe lights, and pitch-blackness are all put to excellent use. The shoulder-mounted lamps worn by most human soldiers are the game's best lighting effect. The lamps cast a cone of white light that points in whichever direction a soldier is looking. The effect is especially good when several guards are creeping around a murky environment, their lamp cones intersecting as they cross paths.
The soundtrack is also excellent. It's a moody mix of ambient clanks and hisses, low bass hums, and screeching strings. The score dynamically changes to a more dramatic composition when you enter a battle. Strangely, it often switches before you're even aware that enemies have detected you, occasionally making the soundtrack a more effective danger signal than the marine's motion detector.
Each of the three campaigns takes about four to five hours to complete, but what each lacks in length more than makes up for in density. A lot of content has been packed into these levels; every one is rich with scripted sequences and little unexpected play elements. As in Monolith's No One Lives Forever, virtually every set of human characters that you encounter can be found engaged in some sort of idle chatter, often about some chaos that you caused while playing as one of the other species. This chatter is especially evident in the Alien and Predator campaigns, which both involve a lot more fights against human opponents and a lot more sneaking around.
Unlike in the original, large portions of Aliens vs. Predator 2 take place outside. The graphics are powered by the latest iteration of Monolith's Lithtech engine, and, while it has a reputation for being technologically somewhat behind the curve, it certainly gets the job done here. If objects occasionally appear a little blocky, the overall art direction--both in terms of its faithfulness to the films and the otherworldly look of the outdoors environments--is beyond reproach. As you stand on a high cliff and watch two perfectly rendered colonial dropships fly by you, bank left, and continue off into the distance, you'll quickly forgive a few too many squared-off desk chairs.
The original Aliens vs. Predator was essentially a series of unrelated levels. It concentrated on creating a mood of relentless dread while relying on the well-developed and well-known Alien and Predator universes to provide an implicit story. For the sequel, however, Monolith has made a complete turnaround. Aliens vs. Predator 2 not only has a plot, but also has one of the most cleverly constructed plots ever attempted in a shooter.
Each of the three seven-level single-player campaigns takes place simultaneously. The events that are made up of one especially bad day for the humans manning a research station on planet LV-1201 are presented from three different perspectives: an alien research subject, a member of a predator hunting party, and a colonial marine who is part of a squad sent in response to a distress beacon. Though each story is self-contained, all three intersect at certain points, and the results of actions in one campaign can be seen in the others. For instance, as the marine, you'll encounter a predator trapped in a cryogenic stasis pod, which you must move so that it can fit down a ventilation shaft. During the predator campaign, it's you in stasis watching as the marine works the pod controls, inadvertently freeing you. The game is filled with little crossovers like these, and it becomes almost like a minigame in itself just keeping track of them, if for no other reason than to appreciate the impressive level of thought that went into creating the story's complex underlying structure.