You can view this a prequel, in which case you might be disappointed that it does not fit the story line exactly. Or you can view it as a movie in its own right, in which case you might think it's entertaining, but not memorable.I've seen all the Alien movies and have them in my collection. I'll add this one, because, whether you think of it as a prequel or not, it's a Ridley Scott movie and he never disappoints.
Pros well paced, great story, directed by ridley scott