Whoever claims that the assassin is 'broken' is simply wrong.
The assassin is arguably the most versatile class in 1.02, and has been so since at least .913
I level my sovereign into an assassin 90% of the time, and I can't complain about the way he performs. The first post of this thread has links to the play-throughs in which assassin sovereigns (and supporting heroes) kick ass. I also plan to level the sovereign in that thread into an assassin, but it has not happened yet - it's still on turn 9.