Will Saves aren't always the answer to a FB
In one of my campaigns, one of the three PCs is a charismatic wild elf Frenzied Berserker who also has two levels in Paladin of Freedom from UA. Thanks to Divine Grace, his Will Save is high enough to render most Will-save based spells useless. I've played with this PC in my campaign for two years now, so I thought I'd share my views on the FB class and how to interact with it. The most potent power of the Frenzied Berserker is the Deathless Frenzy ability, since the FB certainly won't be failing any Fortitude saves. Repeated enemies, who were bothered by his ability to survive at negative several-hundred hit points for long enough to win the fight and then collapse dead, have taken to using Deathwatch and then casting Disintegrate, knowing full well that the FB will save, but ensured that the 5d6 damage will slay him. Another enemy, an evil wizard attempting to break into a fortress who didn't have any disintegrates but had nonetheless done enough damage to kill all the PCs, saluted the Frenzied Berserker's gallant defense to the death and then teleported away. One minute later, he teleported back and strolled in.
Light armour combined with -6 to AC from entering Rage and Frenzy together also make many FBs easy to hit (my campaign's Frenzied Berserker has reached Level 18 through repeated resurrection, but his AC is around 20 in Rage and Frenzy), so remember, turnabout's fair play: use the Power Attack feat against them since your NPCs will probably automatically hit otherwise. The Frenzied Berserker can dish out and receive large amounts of damage, but they tend to get hit again and again. After a few rounds, you may find that the FB has killed the BBEG, but if someone has a disintegrate ready, the FB will fall shortly thereafter. Because of this, it is often a good idea not to throw a big, lone brawn monster as a final encounter against a party with a FB (he and his friends will probably either kill it and live, or the FB will die and then kill it before the frenzy ends), but that's generally good practise even if you don't have a FB. All in all, the Frenzied Berserker was more effective than the Azer Cleric (but as a caster who has taken an ECL 8 race, the azer was hosed to begin with), but the damage-spell-loving wizard consistently dealt more damage, and the NPC cleric, once buffed, could melee for around the same damage or more without getting herself killed.
All that said, these are comments about a Frenzied Berserker as a PC. It seems that Sir Thorncrest was using it as an NPC. Based on the FB dynamic (die while taking some of them with me!), I wouldn't choose to use one against my PCs very often, especially not as a random encounter or winnowing fight. Why not? The FB is probably not going to beat the PCs, but if his saves are high enough to avoid immobilisation and the like, his Deathless Frenzy will probably allow him to fight long enough to kill some of them, so I have provided them with a fight that they certainly can win, but one which will needlessly stall the game as they rush to resurrect the fallen. The one time I used a Frenzied Berserker against the party was a duel with the PC's FB using Merciful weapons, and if it had not been for that, there would have been a PC death.
Hope this helps, and I know that this is anecdotal evidence only. If anyone wants to play the number-crunching game, we can go that route too.