A few thoughts: The 1970s were pretty big on slow, contemplative sci/fi (you can thank Stanley Kubrick for that), and Silent Running is very much in line with that. Set in a future without plant life on Earth, this movie takes place entirely on a space station, largely with only one human character. It's quiet, and the minialist score only adds to that isolation. The only real problem with the music comes from the distractingly hippie-esque recurring songs, but I suppose they make sense within the context of the movie, considering this is a very environmental movie -- I just found them pretty lousy and felt they broke the mood. Bruce Dern is terrific, as always, the cinematography and pacing is excellent, really highlighting the bleak loneliness of the story. It's not a very fun thing to watch, as from beginning to end it's pretty slow and depressing, but it's still a highlight of the genre for the era. Also, it has adorable waddling robots in it, so there's always that to look forward to.
Who would I recommend it to? People who like their sci/fi with more emphasis on mood and aesthetic than on action. It leaves a lot to the imagination.