I've been using TF on my prot warrior since the day I got it. Nowadays, it does seem like a toss-up, but in my personal opinion; I find it to be superior to all tanking weapons.
Keeping up debuffs is a royal pain in the ass while tanking. Not having to worry about one of them alleviates some extra time for more TPS. Sure, an off-tank could do it.. but our warriors do a lot of damage when they aren't tanking, they don't have rage to be wasted on TC. I do think that it is a weapon suited only to MTs nowadays anyways, as the massive amount of rage-free threat it puts out over time adds up to a hefty amount of aggro.
Not only does each proc free up global cooldown and save rage, it also has an additional chance to proc on the devastate that you just used instead of Thunder Clap.
Mobs just seem to stick to me to glue when I'm using Thunderfury. Granted, this is all based on empirical observation and evidence -- but I think most tanks that use TF share the same viewpoint. Maybe when we start getting 110 dps weapons, the sacrifice will be worth it to a select few. But that's a ways off.
We only use 2 tanks on Void Reaver. A Feral Druid and our MT (he also uses thunderfury). No one has pulled aggro off either of them yet.
Is it worth farming if you don't have it? Probably not.
Is it worth using over King's Defender or Blazeguard if you're MT? Most definitely.
We've cleared both T5 instances, so we kinda sorda know what we're doing (not really though).