Axe of Blind Fury is so sweet. Just make sure you get the charge to maximize its usefulness (+1 STR from Furious Charge, +2 attacks from Rage). You're probably a better tactician than I, but sometimes half my bikers (my lord, once) died before getting to swing the dang thing. Choose their route wisely!
I haven't tried any power weapons other than lightning claws on 'zerker champs. People told me there's no reason to take a power sword over the claw, and I find that they're right. Might as well re-roll wounds. Too bad chainaxes sort of suck. With the amount of attacks a squad of 'zerkers gets on the charge, something with a 4+ is probably still going to get shit on.
Anything that doesn't strike at initiative doesn't seem as worth it, imo. A power fist or axe could be useful in taking on a terminator in a challenge, since both will be striking at the same time, but in that situation both will probably die. Not sure if it's worthwhile yet.
Berzerkers + Kharn has always been the winning combo for me. In a Land Raider they're good, in a Dreadclaw they're ridiculous. Gotta wait until turn 3 to charge, but anything you charge is probably going to die, including heavy armor (VRRRRR VRRRRRR die Land Raider!!!! BLOOOOD!!!!) Just make sure the 'zerker champ survives to eat the challenge so that Kharn and the boys can kill everything else. Even if Kharn rolls 1s to hit, he has Hatred so he can re-roll those, so far I haven't lost a berzerker in the 1st turn of combat, and very few combats have lasted more than 1 turn against berzerkers. Either they die or I do. Lightning claw usually guarantees the 'zerker champ wins the challenge, so a roll on the boon table is nice too. Unfortunately, wiping out the enemy squad leaves them open to gunfire, but that's the way it goes with CC units.
Sorry for rambling, I just get excited about Kharn and Berzerkers