Simak also writes about aliens that have an extremely high, almost all knowing, form of intelligence. We're using them, all of them, down to the last hidden corner. In The Skylark of Space , an alien is encountered with a vast intelligence. In both works aliens are represented as being more intellectually evolved than humans. I think that the main strength of Simak's writing is that most of his human characters are very realistic.
Every character has both strengths and flaws.
Simak also tends to focus on the inner struggle that his characters must face. In most of his stories that I read the characters have had to overcome both internal and external obstacles. Simak's characters end up being very interesting and, as a result, so do his stories. The main weakness of Simak's stories is that there is very little use scientific dialogue or description. In that manner he is very much like Ray Bradburry who was writing stories at around the same time.