I think several people will go for it. It has an eight-speed automatic that is supposed to shift very well. It will have the luxury of an AMG, but with a lower price tag. They will take buyers away from the BMW, MB, and audi, since it is the newest thing and many of these buyers are mostly looking for the status symbol/luxury of it.
I really like the looks of the new IS (don't dig the weird quad-pipe exhaust in the IS-F, though), and trust Toyota can do a decent job with the suspension.
I wish I could say more, but I haven't had teh chance to drive one of these.
I think that performance-wise, BMW M is confident in their work. However, in terms of market share, etc., it hurts them every time a car with more power, more luxury, etc., enters the arena.
I really like Toyota as a company. I hope this goes well for them.