gzt: I would expect these dossier allegations to be credible simply because the dossier was written by an experienced ex-spy guy. Whoever paid for that got their money's worth in credible allegations. But credible is not the same as true and may be well crafted but untrue.
Well, the article linked to earlier starts with a careful explanation of what it means to write this kind of report. The dossier is a 'raw' report, which is to say, the author is reporting a true account of what he or she was told. It is not in and of itself necessarily correct. In other words, the author is telling you the truth, but someone may have lied to him.
The article then goes on to say that, as you said above, the report's author is highly experienced and very well regarded by the active intelligence community and furthermore, the FBI paid him money to do further investigation based on some of this report. Therefore it's safe to say that the FBI certainly believe that some of it is credible.