I don't have a problem with them recasting the voices because the guys in suit are just doing performance capture. Although it probably would have been nice to tell them up front. I'm sure it's a bummer for them especially if it's a more recognizable celebrity voice. It's like they run around in those mo-cap suits for a couple of months, and then the A-list celeb goes into the recording studio for a couple of days, enjoys a nice latte, and probably pulls down as much or more (depending on the voice talent) as the actors who did the heavy lifting performance capture. And when the flick comes out the voicing celeb gets the recognition and accolades and is the one signing autographs and walking the red carpet. As someone pointed out earlier, most people associate Vader with James Earl Jones. It's like Shrek (Michael Myers), Donkey (Eddie Murphy), Genie (Robin Williams), etc.
I think the teaser posters look pretty cool. Quite nicely done. While they might be an abomination to many of the fans who grew up on Turtles, I think kids are gonna eat this movie up.