Bat is the hardest fight in the game, but also the most fun. Your adrenalin will be sky-high by the end. Apart from that, you'd be surprised but the loot he drops is very nice, even compared to T4 loot.
T3.5 is a perfect Boot Camp for becoming a hardcore raider.
I imagine once the servers merge, there'll be alot of changes regarding raid progression in the US guilds.
But we digress from the topic of the thread. Don't want to get "infracted".