I couldn't agree more with ntk104 and RuthG--in fact, if he or she was a good employee, you are dealing with someone who has already proven themselves to your company, and, although they left, in some respects must have been a good fit. You know exactly who you are dealing with, and they already know about your work processes, etc., cutting back time on
orientation,
training, etc. So long as they are someone who left on good terms I'd say it's good fortune that they want to come back, making it an easier decision for you/your company than when
hiring someone new.