Is it okay to cite a paper from arXiv? I have found a very good overview of a topic and I want to cite it in the introduction of my paper. The writers are very well known people in the field. Is it then okay to cite the paper that is not published and is only available in arXiv?
There is nothing wrong with citing paper, reviews, etc., which have appeared on the arxiv (and only on the arxiv). Go ahead and cite the paper.
(If your paper gets accepted at a journal, they might ask you to update in case the paper has been published in a journal in the meantime, but in the fields where the arxiv is commonly used, journals and referees will have no issues with references to the arxiv.)