Just to say, "To the Death" can and should be used on lesser mooks as well. Even if you kill them in one hit, every swing during their death animation will generate guard, which means you can always get full guard from it, or use it to tank a group of melee enemies. Of course it won't protect you from archers, but that's how the game goes 
I agree with this. Especially if you are running with a group that is pretty much handling the enemy. Then use To the Death often, just to ensure you are keeping your guard up.
I have promoted the Katari numerous times and I find the class very enjoyable to play, albeit risky.
The builds I have had the most success with is
Charging Bull (+), Mighty Blow (+), Pommel Strike (+) and To the Death (+)
My follow up build is
Charging Bull (+), Mighty Blow(+), Pommel Strike(+) and Earth Shaking Strike(+)
I have tried some of the other skills as well, but Charging Bull and Mighty Blow are just money with the guy. What other players said already I agree with. You have to be selective as to when you first engage a group. I often delay a second and let the Archer/Mages get some licks in before I ever charge. The Katari is not a tank. He is a closer/finisher. You have to keep moving, keep charging and keep enemies knocked down as much as you can once you are engaged.
Whirl Wind is a great fun skill, but is unreliable in MP due to the amount of times it can be blocked and you can get stymied by enemy mobs.
He is a lot of fun, especially on Threatening. To be honest, I do not like playing that class on Perilous, unless I am playing with my selected friends and we are working together.