The psychological effects experienced by the Apollo astronauts are well documented - check out Andy Chaikin's "A Man on the Moon" and Andrew Smith's "Moondust" for two great insights.
Chris Hadfield has also remarked that as well as physical reconditioning he's had to psychologically recondition, even to the simple acts of everyday living that were not present on orbit.
Generally speaking, astronauts are chosen based on psychological stability assessment results and trained to deal with the unusual circumstance of their job. I suspect that any significant long term negative psychological damage would be due more to the proclivity of the individual rather than it being spaceflight itself having a unique psychological effect. I suspect we just don't have enough long term data yet to say if space will make us crazy, but I wonder if there are perhaps studies done on other long term isolation scenarios such as saturation diving or remote outposts (e.g. Antarctica).