In the first story, SUPERMAN Vs. ALIENS, Supes intercepts a signal from wha the believes is a surviving city from Krytpon. He travels there in a spaceship to find the city deserted and lots of dead inhabitants. He also comes across dead crab-like Aliens (Facehuggers!) and the Alien eggs, though he doesn't know what they are at this point. Then, he encounters a full-grown Alien, which manages to cut him. Because Superman is away from the power he gets from the Earth's sun, he's weaker and can be injured. He's rescued by a survivor, a young blonde woman named Kara, and she explains that they are not Kryptonian, just influenced by Krypton. Together, they have to survive and prevent the Aliens from getting to Earth, because people working for Lexcorp wants to use them as a weapon. It's an interesting story, as it sort of re-does the Superman/Supergirl tale.
The second story, SUPERMAN VS ALIENS II: GOD WAR, is much more epic in scope, with Darkseid infecting his warriors with the ALIENS and trying to take over New Genesis. The third tale, THE SUPERMAN/MADMAN HULLABALOO, is strange, as I'm totally unfamiliar with the MADMAN character. The last story may be the most interesting, crossing SUPERMAN and TARZAN. Kal-el's ship crashes in the African Jungle, rather than Smallville, where he's raised by a group of Apes. Eventually he crosses paths with a John Clayton, Lord Greystoke, who never became Tarzan. It's very cool, especially fitting in the Opar story, with La having a chunk of Kryptonite.
I didn't enjoy this graphic novel collection as much as the Batman Aliens book but it's a good read.