People close to Assange have suggested that the reports are credible. A coordinator of the Swedish Wikileaks group called them "very very credible".
However, the credibility of the claims is not important. If Assange is confident that he did not commit the crimes in question and the women's cases are fabrications, then he should not fear returning to Sweden to clear his name. Sweden has an independent judiciary and guarantees defendants a fair trial. He will have the opportunity to fight the charges and if he is innocent of the crimes in question, he will be exonerated.