How bad is it? It's a pretty typical gory Italian horror of the era.
Should you see it? Only if you absolutely love gory Italian horror films.
This film has undoubtedly made lists of worst films just because of the chutzpah of entitling it "Alien 2," when it has no relation to "Alien." The film has a lot of cave exploring - potholing, rather than spelunking, I believe. After a very long dull start, it then gets into full-on gore, mostly involving heads - falling head, exploding head, head sucking - and it's not done particularly well. There's also a lot of dialogue in a bowling alley for a film about aliens.