I will play a Berserker->Cleric over a Fighter/Cleric anyday. Only the Cleric/Ranger can compete with the Berserker->Cleric.
A Kensai->Thief is an interesting anomaly. With the Thief's Use Any Item ability, the Kensai has
NO disadvantage whatsoever. It has decent melee damage and possibly the third-best backstab in the game( beaten only by the Assassin->Fighter and Sarevok the Fighter->Thief).
For great fun, use a Mislead scroll and the Kai ability for dozens of consecutive backstabs of maximum damage cheesiness.