How accurate is the writing of Josephus? Can we rely on any of his writings to establish authentic Jewish practices or in areas of halacha?
The Shulchan Aruch Harav says that one is allowed to read Josephus on Shabbos as it contains words of "Mussar and Yiras Shamayim".
However, the Lubavitcher Rebbe says that
The Lubavitcher Rebbe brings an example where his description of the Beis Hamikdash disagreed with the description in the traditional sources.
However, the Lubavitcher Rebbe pointed out that this could have been a mistranslation into Hebrew.
R. Chaim Knoller (Dvar Yom Beyomo, entry for 17 Iyar) comments, regarding Josephus' sympathetic portrayal of Titus, that as a member of the imperial household he was compelled to falsify history in this regard. (He also gives an interesting homiletic rendering of the phrase in Mikvaos 10:1, "the inkwell of Yosef the kohen had a hole in its side": it hints at the flaw ("hole") in the writings of Josephus, brought about by his living "at the side" of the Romans.)
Doros Harishonim takes an even more hard-line view. He argues that Josephus tendentiously rewrote history in order to portray the Jews as uncivilized rebels, and to whitewash the actions of Agrippa II (in his view, Josephus' collaborator in this effort) that provoked the revolt.