I think they really screwed up with the Final Five story arc, especially with Tigh being a Cylon. It was just totally unbelievable.
Tyrol was bad enough but BSG jumped the shark with Tigh and Ellen, for me. Two of the elder members of the fleet, who within approximate working timelines were youngsters during the first Cylon war when Adama was doing the heroic business; Tigh being a longtime buddy of Adama; displaying variable human ageing characteristics with the other "finals"; flashbacks showing the alternative skinjob concept in its infancy among Cylons at that time; etc.
The justifications introduced for the contradictions and the messy back-story of those five became so convoluted I tired of this whole arc very quickly.
That Five / Temple / Prophecy thing was fundamentally surplus to requirements anyway; for me the whole skinjob thing would have worked so much better if it was just something Cylons did, in the quiet years, to pump out a few young hottie clones like 6 and 8 to infiltrate humanity.
Back when Cylons were sneaky, one step ahead, single minded and dangerous. Not the jarringly contradictory confused schizophrenics in need of some wuv and religious enlightenment of the later series.
I appreciate the need to branch out, but the universe was so rich there were many other avenues they could have branched out into without tearing into the core tenets.