UPDATE 10/29/2014: The above trailer is the full one shown at San Diego Comic Con 2014
I'm not the hugest fan of the way Ultron's final form looks here, but that's a relatively minor quibble with what looks like it'll be another fantastic Marvel movie!
By the way--this trailer was released yesterday, October 22nd. It's already got over 9.3 million views! I'm obviously not the only one who's excited about this movie, due to be released on May 1, 2015! :)
I think this comic book image is a more menacing one, but that's just me...